2020-11-21 01:03:52

by Paul E. McKenney

Subject: [PATCH v2 tip/core/rcu 4/6] srcu: Provide polling interfaces for Tiny SRCU grace periods

From: "Paul E. McKenney" <[email protected]>

There is a need for a polling interface for SRCU grace
periods, so this commit supplies get_state_synchronize_srcu(),
start_poll_synchronize_srcu(), and poll_state_synchronize_srcu() for this
purpose. The first can be used if future grace periods are inevitable
(perhaps due to a later call_srcu() invocation), the second if future
grace periods might not otherwise happen, and the third to check if a
grace period has elapsed since the corresponding call to either of the
first two.

As with get_state_synchronize_rcu() and cond_synchronize_rcu(),
the return value from either get_state_synchronize_srcu() or
start_poll_synchronize_srcu() must be passed in to a later call to
poll_state_synchronize_srcu().

Link: https://lore.kernel.org/rcu/[email protected]/
Reported-by: Kent Overstreet <[email protected]>
[ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ]
[ paulmck: Apply feedback from Neeraj Upadhyay. ]
Link: https://lore.kernel.org/lkml/20201117004017.GA7444@paulmck-ThinkPad-P72/
Signed-off-by: Paul E. McKenney <[email protected]>
---
include/linux/rcupdate.h | 2 ++
include/linux/srcu.h | 3 +++
include/linux/srcutiny.h | 1 +
kernel/rcu/srcutiny.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++--
4 files changed, 56 insertions(+), 2 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index de08264..e09c0d8 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -33,6 +33,8 @@
#define ULONG_CMP_GE(a, b) (ULONG_MAX / 2 >= (a) - (b))
#define ULONG_CMP_LT(a, b) (ULONG_MAX / 2 < (a) - (b))
#define ulong2long(a) (*(long *)(&(a)))
+#define USHORT_CMP_GE(a, b) (USHRT_MAX / 2 >= (unsigned short)((a) - (b)))
+#define USHORT_CMP_LT(a, b) (USHRT_MAX / 2 < (unsigned short)((a) - (b)))

/* Exported common interfaces */
void call_rcu(struct rcu_head *head, rcu_callback_t func);
diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index e432cc9..a0895bb 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -60,6 +60,9 @@ void cleanup_srcu_struct(struct srcu_struct *ssp);
int __srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp);
void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp);
void synchronize_srcu(struct srcu_struct *ssp);
+unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp);
+unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp);
+bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie);

#ifdef CONFIG_DEBUG_LOCK_ALLOC

diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
index d9edb67..c7f0c1f 100644
--- a/include/linux/srcutiny.h
+++ b/include/linux/srcutiny.h
@@ -16,6 +16,7 @@
struct srcu_struct {
short srcu_lock_nesting[2]; /* srcu_read_lock() nesting depth. */
unsigned short srcu_idx; /* Current reader array element in bit 0x2. */
+ unsigned short srcu_idx_max; /* Furthest future srcu_idx request. */
u8 srcu_gp_running; /* GP workqueue running? */
u8 srcu_gp_waiting; /* GP waiting for readers? */
struct swait_queue_head srcu_wq;
diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
index 3bac1db..b073175 100644
--- a/kernel/rcu/srcutiny.c
+++ b/kernel/rcu/srcutiny.c
@@ -34,6 +34,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp)
ssp->srcu_gp_running = false;
ssp->srcu_gp_waiting = false;
ssp->srcu_idx = 0;
+ ssp->srcu_idx_max = 0;
INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
INIT_LIST_HEAD(&ssp->srcu_work.entry);
return 0;
@@ -114,7 +115,7 @@ void srcu_drive_gp(struct work_struct *wp)
struct srcu_struct *ssp;

ssp = container_of(wp, struct srcu_struct, srcu_work);
- if (ssp->srcu_gp_running || !READ_ONCE(ssp->srcu_cb_head))
+ if (ssp->srcu_gp_running || USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
return; /* Already running or nothing to do. */

/* Remove recently arrived callbacks and wait for readers. */
@@ -147,13 +148,18 @@ void srcu_drive_gp(struct work_struct *wp)
* straighten that out.
*/
WRITE_ONCE(ssp->srcu_gp_running, false);
- if (READ_ONCE(ssp->srcu_cb_head))
+ if (USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
schedule_work(&ssp->srcu_work);
}
EXPORT_SYMBOL_GPL(srcu_drive_gp);

static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
{
+ unsigned short cookie;
+
+ cookie = get_state_synchronize_srcu(ssp);
+ if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
+ WRITE_ONCE(ssp->srcu_idx_max, cookie);
if (!READ_ONCE(ssp->srcu_gp_running)) {
if (likely(srcu_init_done))
schedule_work(&ssp->srcu_work);
@@ -196,6 +202,48 @@ void synchronize_srcu(struct srcu_struct *ssp)
}
EXPORT_SYMBOL_GPL(synchronize_srcu);

+/*
+ * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
+ */
+unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
+{
+ unsigned long ret;
+
+ barrier();
+ ret = (READ_ONCE(ssp->srcu_idx) + 3) & ~0x1;
+ barrier();
+ return ret & USHRT_MAX;
+}
+EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
+
+/*
+ * start_poll_synchronize_srcu - Provide cookie and start grace period
+ *
+ * The difference between this and get_state_synchronize_srcu() is that
+ * this function ensures that the poll_state_synchronize_srcu() will
+ * eventually return the value true.
+ */
+unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
+{
+ unsigned long ret = get_state_synchronize_srcu(ssp);
+
+ srcu_gp_start_if_needed(ssp);
+ return ret;
+}
+EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
+
+/*
+ * poll_state_synchronize_srcu - Has cookie's grace period ended?
+ */
+bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
+{
+ bool ret = USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx), cookie);
+
+ barrier();
+ return ret;
+}
+EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
+
/* Lockdep diagnostics. */
void __init rcu_scheduler_starting(void)
{
--
2.9.5

2020-11-22 14:35:23

by Neeraj Upadhyay

Subject: Re: [PATCH v2 tip/core/rcu 4/6] srcu: Provide polling interfaces for Tiny SRCU grace periods

On 11/21/2020 6:29 AM, [email protected] wrote:
> From: "Paul E. McKenney" <[email protected]>
>
> There is a need for a polling interface for SRCU grace
> periods, so this commit supplies get_state_synchronize_srcu(),
> start_poll_synchronize_srcu(), and poll_state_synchronize_srcu() for this
> purpose. The first can be used if future grace periods are inevitable
> (perhaps due to a later call_srcu() invocation), the second if future
> grace periods might not otherwise happen, and the third to check if a
> grace period has elapsed since the corresponding call to either of the
> first two.
>
> As with get_state_synchronize_rcu() and cond_synchronize_rcu(),
> the return value from either get_state_synchronize_srcu() or
> start_poll_synchronize_srcu() must be passed in to a later call to
> poll_state_synchronize_srcu().
>
> Link: https://lore.kernel.org/rcu/[email protected]/
> Reported-by: Kent Overstreet <[email protected]>
> [ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ]
> [ paulmck: Apply feedback from Neeraj Upadhyay. ]
> Link: https://lore.kernel.org/lkml/20201117004017.GA7444@paulmck-ThinkPad-P72/
> Signed-off-by: Paul E. McKenney <[email protected]>
> ---
> include/linux/rcupdate.h | 2 ++
> include/linux/srcu.h | 3 +++
> include/linux/srcutiny.h | 1 +
> kernel/rcu/srcutiny.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++--
> 4 files changed, 56 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index de08264..e09c0d8 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -33,6 +33,8 @@
> #define ULONG_CMP_GE(a, b) (ULONG_MAX / 2 >= (a) - (b))
> #define ULONG_CMP_LT(a, b) (ULONG_MAX / 2 < (a) - (b))
> #define ulong2long(a) (*(long *)(&(a)))
> +#define USHORT_CMP_GE(a, b) (USHRT_MAX / 2 >= (unsigned short)((a) - (b)))
> +#define USHORT_CMP_LT(a, b) (USHRT_MAX / 2 < (unsigned short)((a) - (b)))
>
> /* Exported common interfaces */
> void call_rcu(struct rcu_head *head, rcu_callback_t func);
> diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> index e432cc9..a0895bb 100644
> --- a/include/linux/srcu.h
> +++ b/include/linux/srcu.h
> @@ -60,6 +60,9 @@ void cleanup_srcu_struct(struct srcu_struct *ssp);
> int __srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp);
> void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp);
> void synchronize_srcu(struct srcu_struct *ssp);
> +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp);
> +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp);
> +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie);
>
> #ifdef CONFIG_DEBUG_LOCK_ALLOC
>
> diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
> index d9edb67..c7f0c1f 100644
> --- a/include/linux/srcutiny.h
> +++ b/include/linux/srcutiny.h
> @@ -16,6 +16,7 @@
> struct srcu_struct {
> short srcu_lock_nesting[2]; /* srcu_read_lock() nesting depth. */
> unsigned short srcu_idx; /* Current reader array element in bit 0x2. */
> + unsigned short srcu_idx_max; /* Furthest future srcu_idx request. */
> u8 srcu_gp_running; /* GP workqueue running? */
> u8 srcu_gp_waiting; /* GP waiting for readers? */
> struct swait_queue_head srcu_wq;
> diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
> index 3bac1db..b073175 100644
> --- a/kernel/rcu/srcutiny.c
> +++ b/kernel/rcu/srcutiny.c
> @@ -34,6 +34,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp)
> ssp->srcu_gp_running = false;
> ssp->srcu_gp_waiting = false;
> ssp->srcu_idx = 0;
> + ssp->srcu_idx_max = 0;
> INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
> INIT_LIST_HEAD(&ssp->srcu_work.entry);
> return 0;
> @@ -114,7 +115,7 @@ void srcu_drive_gp(struct work_struct *wp)
> struct srcu_struct *ssp;
>
> ssp = container_of(wp, struct srcu_struct, srcu_work);
> - if (ssp->srcu_gp_running || !READ_ONCE(ssp->srcu_cb_head))
> + if (ssp->srcu_gp_running || USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
> return; /* Already running or nothing to do. */
>
> /* Remove recently arrived callbacks and wait for readers. */
> @@ -147,13 +148,18 @@ void srcu_drive_gp(struct work_struct *wp)
> * straighten that out.
> */
> WRITE_ONCE(ssp->srcu_gp_running, false);
> - if (READ_ONCE(ssp->srcu_cb_head))
> + if (USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))

USHORT_CMP_LT ?

Thanks
Neeraj

> schedule_work(&ssp->srcu_work);

> }
> EXPORT_SYMBOL_GPL(srcu_drive_gp);
>
> static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
> {
> + unsigned short cookie;
> +
> + cookie = get_state_synchronize_srcu(ssp);
> + if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
> + WRITE_ONCE(ssp->srcu_idx_max, cookie);
> if (!READ_ONCE(ssp->srcu_gp_running)) {
> if (likely(srcu_init_done))
> schedule_work(&ssp->srcu_work);
> @@ -196,6 +202,48 @@ void synchronize_srcu(struct srcu_struct *ssp)
> }
> EXPORT_SYMBOL_GPL(synchronize_srcu);
>
> +/*
> + * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
> + */
> +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
> +{
> + unsigned long ret;
> +
> + barrier();
> + ret = (READ_ONCE(ssp->srcu_idx) + 3) & ~0x1;
> + barrier();
> + return ret & USHRT_MAX;
> +}
> +EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
> +
> +/*
> + * start_poll_synchronize_srcu - Provide cookie and start grace period
> + *
> + * The difference between this and get_state_synchronize_srcu() is that
> + * this function ensures that the poll_state_synchronize_srcu() will
> + * eventually return the value true.
> + */
> +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
> +{
> + unsigned long ret = get_state_synchronize_srcu(ssp);
> +
> + srcu_gp_start_if_needed(ssp);
> + return ret;
> +}
> +EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
> +
> +/*
> + * poll_state_synchronize_srcu - Has cookie's grace period ended?
> + */
> +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
> +{
> + bool ret = USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx), cookie);
> +
> + barrier();
> + return ret;
> +}
> +EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
> +
> /* Lockdep diagnostics. */
> void __init rcu_scheduler_starting(void)
> {
>

--
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
member of the Code Aurora Forum, hosted by The Linux Foundation

2020-11-22 18:15:34

by Paul E. McKenney

Subject: Re: [PATCH v2 tip/core/rcu 4/6] srcu: Provide polling interfaces for Tiny SRCU grace periods

On Sun, Nov 22, 2020 at 08:00:09PM +0530, Neeraj Upadhyay wrote:
>
>
> On 11/21/2020 6:29 AM, [email protected] wrote:
> > From: "Paul E. McKenney" <[email protected]>
> >
> > There is a need for a polling interface for SRCU grace
> > periods, so this commit supplies get_state_synchronize_srcu(),
> > start_poll_synchronize_srcu(), and poll_state_synchronize_srcu() for this
> > purpose. The first can be used if future grace periods are inevitable
> > (perhaps due to a later call_srcu() invocation), the second if future
> > grace periods might not otherwise happen, and the third to check if a
> > grace period has elapsed since the corresponding call to either of the
> > first two.
> >
> > As with get_state_synchronize_rcu() and cond_synchronize_rcu(),
> > the return value from either get_state_synchronize_srcu() or
> > start_poll_synchronize_srcu() must be passed in to a later call to
> > poll_state_synchronize_srcu().
> >
> > Link: https://lore.kernel.org/rcu/[email protected]/
> > Reported-by: Kent Overstreet <[email protected]>
> > [ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ]
> > [ paulmck: Apply feedback from Neeraj Upadhyay. ]
> > Link: https://lore.kernel.org/lkml/20201117004017.GA7444@paulmck-ThinkPad-P72/
> > Signed-off-by: Paul E. McKenney <[email protected]>
> > ---
> > include/linux/rcupdate.h | 2 ++
> > include/linux/srcu.h | 3 +++
> > include/linux/srcutiny.h | 1 +
> > kernel/rcu/srcutiny.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++--
> > 4 files changed, 56 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index de08264..e09c0d8 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -33,6 +33,8 @@
> > #define ULONG_CMP_GE(a, b) (ULONG_MAX / 2 >= (a) - (b))
> > #define ULONG_CMP_LT(a, b) (ULONG_MAX / 2 < (a) - (b))
> > #define ulong2long(a) (*(long *)(&(a)))
> > +#define USHORT_CMP_GE(a, b) (USHRT_MAX / 2 >= (unsigned short)((a) - (b)))
> > +#define USHORT_CMP_LT(a, b) (USHRT_MAX / 2 < (unsigned short)((a) - (b)))
> > /* Exported common interfaces */
> > void call_rcu(struct rcu_head *head, rcu_callback_t func);
> > diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> > index e432cc9..a0895bb 100644
> > --- a/include/linux/srcu.h
> > +++ b/include/linux/srcu.h
> > @@ -60,6 +60,9 @@ void cleanup_srcu_struct(struct srcu_struct *ssp);
> > int __srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp);
> > void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp);
> > void synchronize_srcu(struct srcu_struct *ssp);
> > +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp);
> > +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp);
> > +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie);
> > #ifdef CONFIG_DEBUG_LOCK_ALLOC
> > diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
> > index d9edb67..c7f0c1f 100644
> > --- a/include/linux/srcutiny.h
> > +++ b/include/linux/srcutiny.h
> > @@ -16,6 +16,7 @@
> > struct srcu_struct {
> > short srcu_lock_nesting[2]; /* srcu_read_lock() nesting depth. */
> > unsigned short srcu_idx; /* Current reader array element in bit 0x2. */
> > + unsigned short srcu_idx_max; /* Furthest future srcu_idx request. */
> > u8 srcu_gp_running; /* GP workqueue running? */
> > u8 srcu_gp_waiting; /* GP waiting for readers? */
> > struct swait_queue_head srcu_wq;
> > diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
> > index 3bac1db..b073175 100644
> > --- a/kernel/rcu/srcutiny.c
> > +++ b/kernel/rcu/srcutiny.c
> > @@ -34,6 +34,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp)
> > ssp->srcu_gp_running = false;
> > ssp->srcu_gp_waiting = false;
> > ssp->srcu_idx = 0;
> > + ssp->srcu_idx_max = 0;
> > INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
> > INIT_LIST_HEAD(&ssp->srcu_work.entry);
> > return 0;
> > @@ -114,7 +115,7 @@ void srcu_drive_gp(struct work_struct *wp)
> > struct srcu_struct *ssp;
> > ssp = container_of(wp, struct srcu_struct, srcu_work);
> > - if (ssp->srcu_gp_running || !READ_ONCE(ssp->srcu_cb_head))
> > + if (ssp->srcu_gp_running || USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
> > return; /* Already running or nothing to do. */
> > /* Remove recently arrived callbacks and wait for readers. */
> > @@ -147,13 +148,18 @@ void srcu_drive_gp(struct work_struct *wp)
> > * straighten that out.
> > */
> > WRITE_ONCE(ssp->srcu_gp_running, false);
> > - if (READ_ONCE(ssp->srcu_cb_head))
> > + if (USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
>
> USHORT_CMP_LT ?

Gah! I squashed that change with 5/6 rather than 4/6. I will adjust,
and thank you for checking!

Thanx, Paul

> Thanks
> Neeraj
>
> > schedule_work(&ssp->srcu_work);
>
> > }
> > EXPORT_SYMBOL_GPL(srcu_drive_gp);
> > static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
> > {
> > + unsigned short cookie;
> > +
> > + cookie = get_state_synchronize_srcu(ssp);
> > + if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
> > + WRITE_ONCE(ssp->srcu_idx_max, cookie);
> > if (!READ_ONCE(ssp->srcu_gp_running)) {
> > if (likely(srcu_init_done))
> > schedule_work(&ssp->srcu_work);
> > @@ -196,6 +202,48 @@ void synchronize_srcu(struct srcu_struct *ssp)
> > }
> > EXPORT_SYMBOL_GPL(synchronize_srcu);
> > +/*
> > + * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
> > + */
> > +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
> > +{
> > + unsigned long ret;
> > +
> > + barrier();
> > + ret = (READ_ONCE(ssp->srcu_idx) + 3) & ~0x1;
> > + barrier();
> > + return ret & USHRT_MAX;
> > +}
> > +EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
> > +
> > +/*
> > + * start_poll_synchronize_srcu - Provide cookie and start grace period
> > + *
> > + * The difference between this and get_state_synchronize_srcu() is that
> > + * this function ensures that the poll_state_synchronize_srcu() will
> > + * eventually return the value true.
> > + */
> > +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
> > +{
> > + unsigned long ret = get_state_synchronize_srcu(ssp);
> > +
> > + srcu_gp_start_if_needed(ssp);
> > + return ret;
> > +}
> > +EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
> > +
> > +/*
> > + * poll_state_synchronize_srcu - Has cookie's grace period ended?
> > + */
> > +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
> > +{
> > + bool ret = USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx), cookie);
> > +
> > + barrier();
> > + return ret;
> > +}
> > +EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
> > +
> > /* Lockdep diagnostics. */
> > void __init rcu_scheduler_starting(void)
> > {
> >
>
> --
> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of
> the Code Aurora Forum, hosted by The Linux Foundation

2020-11-23 04:50:01

by Neeraj Upadhyay

Subject: Re: [PATCH v2 tip/core/rcu 4/6] srcu: Provide polling interfaces for Tiny SRCU grace periods

On 11/21/2020 6:29 AM, [email protected] wrote:
> From: "Paul E. McKenney" <[email protected]>
>
> There is a need for a polling interface for SRCU grace
> periods, so this commit supplies get_state_synchronize_srcu(),
> start_poll_synchronize_srcu(), and poll_state_synchronize_srcu() for this
> purpose. The first can be used if future grace periods are inevitable
> (perhaps due to a later call_srcu() invocation), the second if future
> grace periods might not otherwise happen, and the third to check if a
> grace period has elapsed since the corresponding call to either of the
> first two.
>
> As with get_state_synchronize_rcu() and cond_synchronize_rcu(),
> the return value from either get_state_synchronize_srcu() or
> start_poll_synchronize_srcu() must be passed in to a later call to
> poll_state_synchronize_srcu().
>
> Link: https://lore.kernel.org/rcu/[email protected]/
> Reported-by: Kent Overstreet <[email protected]>
> [ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ]
> [ paulmck: Apply feedback from Neeraj Upadhyay. ]
> Link: https://lore.kernel.org/lkml/20201117004017.GA7444@paulmck-ThinkPad-P72/
> Signed-off-by: Paul E. McKenney <[email protected]>
> ---
> include/linux/rcupdate.h | 2 ++
> include/linux/srcu.h | 3 +++
> include/linux/srcutiny.h | 1 +
> kernel/rcu/srcutiny.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++--
> 4 files changed, 56 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index de08264..e09c0d8 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -33,6 +33,8 @@
> #define ULONG_CMP_GE(a, b) (ULONG_MAX / 2 >= (a) - (b))
> #define ULONG_CMP_LT(a, b) (ULONG_MAX / 2 < (a) - (b))
> #define ulong2long(a) (*(long *)(&(a)))
> +#define USHORT_CMP_GE(a, b) (USHRT_MAX / 2 >= (unsigned short)((a) - (b)))
> +#define USHORT_CMP_LT(a, b) (USHRT_MAX / 2 < (unsigned short)((a) - (b)))
>
> /* Exported common interfaces */
> void call_rcu(struct rcu_head *head, rcu_callback_t func);
> diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> index e432cc9..a0895bb 100644
> --- a/include/linux/srcu.h
> +++ b/include/linux/srcu.h
> @@ -60,6 +60,9 @@ void cleanup_srcu_struct(struct srcu_struct *ssp);
> int __srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp);
> void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp);
> void synchronize_srcu(struct srcu_struct *ssp);
> +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp);
> +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp);
> +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie);
>
> #ifdef CONFIG_DEBUG_LOCK_ALLOC
>
> diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
> index d9edb67..c7f0c1f 100644
> --- a/include/linux/srcutiny.h
> +++ b/include/linux/srcutiny.h
> @@ -16,6 +16,7 @@
> struct srcu_struct {
> short srcu_lock_nesting[2]; /* srcu_read_lock() nesting depth. */
> unsigned short srcu_idx; /* Current reader array element in bit 0x2. */
> + unsigned short srcu_idx_max; /* Furthest future srcu_idx request. */
> u8 srcu_gp_running; /* GP workqueue running? */
> u8 srcu_gp_waiting; /* GP waiting for readers? */
> struct swait_queue_head srcu_wq;
> diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
> index 3bac1db..b073175 100644
> --- a/kernel/rcu/srcutiny.c
> +++ b/kernel/rcu/srcutiny.c
> @@ -34,6 +34,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp)
> ssp->srcu_gp_running = false;
> ssp->srcu_gp_waiting = false;
> ssp->srcu_idx = 0;
> + ssp->srcu_idx_max = 0;
> INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
> INIT_LIST_HEAD(&ssp->srcu_work.entry);
> return 0;

Minor: cleanup_srcu_struct() can probably have 2 new sanity checks?

WARN_ON(ssp->srcu_idx != ssp->srcu_idx_max);
WARN_ON(ssp->srcu_idx & 1);

Thanks
Neeraj

> @@ -114,7 +115,7 @@ void srcu_drive_gp(struct work_struct *wp)
> struct srcu_struct *ssp;
>
> ssp = container_of(wp, struct srcu_struct, srcu_work);
> - if (ssp->srcu_gp_running || !READ_ONCE(ssp->srcu_cb_head))
> + if (ssp->srcu_gp_running || USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
> return; /* Already running or nothing to do. */
>
> /* Remove recently arrived callbacks and wait for readers. */
> @@ -147,13 +148,18 @@ void srcu_drive_gp(struct work_struct *wp)
> * straighten that out.
> */
> WRITE_ONCE(ssp->srcu_gp_running, false);
> - if (READ_ONCE(ssp->srcu_cb_head))
> + if (USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
> schedule_work(&ssp->srcu_work);
> }
> EXPORT_SYMBOL_GPL(srcu_drive_gp);
>
> static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
> {
> + unsigned short cookie;
> +
> + cookie = get_state_synchronize_srcu(ssp);
> + if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
> + WRITE_ONCE(ssp->srcu_idx_max, cookie);

Minor: Maybe we can return in the else part of USHORT_CMP_LT check, to
avoid scheduling work?

Thanks
Neeraj

> if (!READ_ONCE(ssp->srcu_gp_running)) {
> if (likely(srcu_init_done))
> schedule_work(&ssp->srcu_work);
> @@ -196,6 +202,48 @@ void synchronize_srcu(struct srcu_struct *ssp)
> }
> EXPORT_SYMBOL_GPL(synchronize_srcu);
>
> +/*
> + * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
> + */
> +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
> +{
> + unsigned long ret;
> +
> + barrier();
> + ret = (READ_ONCE(ssp->srcu_idx) + 3) & ~0x1;
> + barrier();
> + return ret & USHRT_MAX;
> +}
> +EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
> +
> +/*
> + * start_poll_synchronize_srcu - Provide cookie and start grace period
> + *
> + * The difference between this and get_state_synchronize_srcu() is that
> + * this function ensures that the poll_state_synchronize_srcu() will
> + * eventually return the value true.
> + */
> +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
> +{
> + unsigned long ret = get_state_synchronize_srcu(ssp);
> +
> + srcu_gp_start_if_needed(ssp);
> + return ret;
> +}
> +EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
> +
> +/*
> + * poll_state_synchronize_srcu - Has cookie's grace period ended?
> + */
> +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
> +{
> + bool ret = USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx), cookie);
> +
> + barrier();
> + return ret;
> +}
> +EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
> +
> /* Lockdep diagnostics. */
> void __init rcu_scheduler_starting(void)
> {
>

--
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
member of the Code Aurora Forum, hosted by The Linux Foundation

2020-11-23 21:17:15

by Paul E. McKenney

Subject: Re: [PATCH v2 tip/core/rcu 4/6] srcu: Provide polling interfaces for Tiny SRCU grace periods

On Mon, Nov 23, 2020 at 10:13:13AM +0530, Neeraj Upadhyay wrote:
>
>
> On 11/21/2020 6:29 AM, [email protected] wrote:
> > From: "Paul E. McKenney" <[email protected]>
> >
> > There is a need for a polling interface for SRCU grace
> > periods, so this commit supplies get_state_synchronize_srcu(),
> > start_poll_synchronize_srcu(), and poll_state_synchronize_srcu() for this
> > purpose. The first can be used if future grace periods are inevitable
> > (perhaps due to a later call_srcu() invocation), the second if future
> > grace periods might not otherwise happen, and the third to check if a
> > grace period has elapsed since the corresponding call to either of the
> > first two.
> >
> > As with get_state_synchronize_rcu() and cond_synchronize_rcu(),
> > the return value from either get_state_synchronize_srcu() or
> > start_poll_synchronize_srcu() must be passed in to a later call to
> > poll_state_synchronize_srcu().
> >
> > Link: https://lore.kernel.org/rcu/[email protected]/
> > Reported-by: Kent Overstreet <[email protected]>
> > [ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ]
> > [ paulmck: Apply feedback from Neeraj Upadhyay. ]
> > Link: https://lore.kernel.org/lkml/20201117004017.GA7444@paulmck-ThinkPad-P72/
> > Signed-off-by: Paul E. McKenney <[email protected]>
> > ---
> > include/linux/rcupdate.h | 2 ++
> > include/linux/srcu.h | 3 +++
> > include/linux/srcutiny.h | 1 +
> > kernel/rcu/srcutiny.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++--
> > 4 files changed, 56 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index de08264..e09c0d8 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -33,6 +33,8 @@
> > #define ULONG_CMP_GE(a, b) (ULONG_MAX / 2 >= (a) - (b))
> > #define ULONG_CMP_LT(a, b) (ULONG_MAX / 2 < (a) - (b))
> > #define ulong2long(a) (*(long *)(&(a)))
> > +#define USHORT_CMP_GE(a, b) (USHRT_MAX / 2 >= (unsigned short)((a) - (b)))
> > +#define USHORT_CMP_LT(a, b) (USHRT_MAX / 2 < (unsigned short)((a) - (b)))
> > /* Exported common interfaces */
> > void call_rcu(struct rcu_head *head, rcu_callback_t func);
> > diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> > index e432cc9..a0895bb 100644
> > --- a/include/linux/srcu.h
> > +++ b/include/linux/srcu.h
> > @@ -60,6 +60,9 @@ void cleanup_srcu_struct(struct srcu_struct *ssp);
> > int __srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp);
> > void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp);
> > void synchronize_srcu(struct srcu_struct *ssp);
> > +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp);
> > +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp);
> > +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie);
> > #ifdef CONFIG_DEBUG_LOCK_ALLOC
> > diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
> > index d9edb67..c7f0c1f 100644
> > --- a/include/linux/srcutiny.h
> > +++ b/include/linux/srcutiny.h
> > @@ -16,6 +16,7 @@
> > struct srcu_struct {
> > short srcu_lock_nesting[2]; /* srcu_read_lock() nesting depth. */
> > unsigned short srcu_idx; /* Current reader array element in bit 0x2. */
> > + unsigned short srcu_idx_max; /* Furthest future srcu_idx request. */
> > u8 srcu_gp_running; /* GP workqueue running? */
> > u8 srcu_gp_waiting; /* GP waiting for readers? */
> > struct swait_queue_head srcu_wq;
> > diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
> > index 3bac1db..b073175 100644
> > --- a/kernel/rcu/srcutiny.c
> > +++ b/kernel/rcu/srcutiny.c
> > @@ -34,6 +34,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp)
> > ssp->srcu_gp_running = false;
> > ssp->srcu_gp_waiting = false;
> > ssp->srcu_idx = 0;
> > + ssp->srcu_idx_max = 0;
> > INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
> > INIT_LIST_HEAD(&ssp->srcu_work.entry);
> > return 0;
>
> Minor: cleanup_srcu_struct() can probably have 2 new sanity checks?
>
> WARN_ON(ssp->srcu_idx != ssp->srcu_idx_max);
> WARN_ON(ssp->srcu_idx & 1);

Good point, added and under test.

> Thanks
> Neeraj
>
> > @@ -114,7 +115,7 @@ void srcu_drive_gp(struct work_struct *wp)
> > struct srcu_struct *ssp;
> > ssp = container_of(wp, struct srcu_struct, srcu_work);
> > - if (ssp->srcu_gp_running || !READ_ONCE(ssp->srcu_cb_head))
> > + if (ssp->srcu_gp_running || USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
> > return; /* Already running or nothing to do. */
> > /* Remove recently arrived callbacks and wait for readers. */
> > @@ -147,13 +148,18 @@ void srcu_drive_gp(struct work_struct *wp)
> > * straighten that out.
> > */
> > WRITE_ONCE(ssp->srcu_gp_running, false);
> > - if (READ_ONCE(ssp->srcu_cb_head))
> > + if (USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
> > schedule_work(&ssp->srcu_work);
> > }
> > EXPORT_SYMBOL_GPL(srcu_drive_gp);
> > static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
> > {
> > + unsigned short cookie;
> > +
> > + cookie = get_state_synchronize_srcu(ssp);
> > + if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
> > + WRITE_ONCE(ssp->srcu_idx_max, cookie);
>
> Minor: Maybe we can return in the else part of USHORT_CMP_LT check, to avoid
> scheduling work?

How about like this?

static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
{
unsigned short cookie;

cookie = get_state_synchronize_srcu(ssp);
if (USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx_max), cookie))
return;
WRITE_ONCE(ssp->srcu_idx_max, cookie);
if (!READ_ONCE(ssp->srcu_gp_running)) {
if (likely(srcu_init_done))
schedule_work(&ssp->srcu_work);
else if (list_empty(&ssp->srcu_work.entry))
list_add(&ssp->srcu_work.entry, &srcu_boot_list);
}
}

Testing this next. ;-)

Thanx, Paul

> Thanks
> Neeraj
>
> > if (!READ_ONCE(ssp->srcu_gp_running)) {
> > if (likely(srcu_init_done))
> > schedule_work(&ssp->srcu_work);
> > @@ -196,6 +202,48 @@ void synchronize_srcu(struct srcu_struct *ssp)
> > }
> > EXPORT_SYMBOL_GPL(synchronize_srcu);
> > +/*
> > + * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
> > + */
> > +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
> > +{
> > + unsigned long ret;
> > +
> > + barrier();
> > + ret = (READ_ONCE(ssp->srcu_idx) + 3) & ~0x1;
> > + barrier();
> > + return ret & USHRT_MAX;
> > +}
> > +EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
> > +
> > +/*
> > + * start_poll_synchronize_srcu - Provide cookie and start grace period
> > + *
> > + * The difference between this and get_state_synchronize_srcu() is that
> > + * this function ensures that the poll_state_synchronize_srcu() will
> > + * eventually return the value true.
> > + */
> > +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
> > +{
> > + unsigned long ret = get_state_synchronize_srcu(ssp);
> > +
> > + srcu_gp_start_if_needed(ssp);
> > + return ret;
> > +}
> > +EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
> > +
> > +/*
> > + * poll_state_synchronize_srcu - Has cookie's grace period ended?
> > + */
> > +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
> > +{
> > + bool ret = USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx), cookie);
> > +
> > + barrier();
> > + return ret;
> > +}
> > +EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
> > +
> > /* Lockdep diagnostics. */
> > void __init rcu_scheduler_starting(void)
> > {
> >
>
> --
> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of
> the Code Aurora Forum, hosted by The Linux Foundation

2020-11-24 05:21:46

by Neeraj Upadhyay

Subject: Re: [PATCH v2 tip/core/rcu 4/6] srcu: Provide polling interfaces for Tiny SRCU grace periods

On 11/24/2020 2:42 AM, Paul E. McKenney wrote:
> On Mon, Nov 23, 2020 at 10:13:13AM +0530, Neeraj Upadhyay wrote:
>>
>>
>> On 11/21/2020 6:29 AM, [email protected] wrote:
>>> From: "Paul E. McKenney" <[email protected]>
>>>
>>> There is a need for a polling interface for SRCU grace
>>> periods, so this commit supplies get_state_synchronize_srcu(),
>>> start_poll_synchronize_srcu(), and poll_state_synchronize_srcu() for this
>>> purpose. The first can be used if future grace periods are inevitable
>>> (perhaps due to a later call_srcu() invocation), the second if future
>>> grace periods might not otherwise happen, and the third to check if a
>>> grace period has elapsed since the corresponding call to either of the
>>> first two.
>>>
>>> As with get_state_synchronize_rcu() and cond_synchronize_rcu(),
>>> the return value from either get_state_synchronize_srcu() or
>>> start_poll_synchronize_srcu() must be passed in to a later call to
>>> poll_state_synchronize_srcu().
>>>
>>> Link: https://lore.kernel.org/rcu/[email protected]/
>>> Reported-by: Kent Overstreet <[email protected]>
>>> [ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ]
>>> [ paulmck: Apply feedback from Neeraj Upadhyay. ]
>>> Link: https://lore.kernel.org/lkml/20201117004017.GA7444@paulmck-ThinkPad-P72/
>>> Signed-off-by: Paul E. McKenney <[email protected]>
>>> ---
>>> include/linux/rcupdate.h | 2 ++
>>> include/linux/srcu.h | 3 +++
>>> include/linux/srcutiny.h | 1 +
>>> kernel/rcu/srcutiny.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++--
>>> 4 files changed, 56 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
>>> index de08264..e09c0d8 100644
>>> --- a/include/linux/rcupdate.h
>>> +++ b/include/linux/rcupdate.h
>>> @@ -33,6 +33,8 @@
>>> #define ULONG_CMP_GE(a, b) (ULONG_MAX / 2 >= (a) - (b))
>>> #define ULONG_CMP_LT(a, b) (ULONG_MAX / 2 < (a) - (b))
>>> #define ulong2long(a) (*(long *)(&(a)))
>>> +#define USHORT_CMP_GE(a, b) (USHRT_MAX / 2 >= (unsigned short)((a) - (b)))
>>> +#define USHORT_CMP_LT(a, b) (USHRT_MAX / 2 < (unsigned short)((a) - (b)))
>>> /* Exported common interfaces */
>>> void call_rcu(struct rcu_head *head, rcu_callback_t func);
>>> diff --git a/include/linux/srcu.h b/include/linux/srcu.h
>>> index e432cc9..a0895bb 100644
>>> --- a/include/linux/srcu.h
>>> +++ b/include/linux/srcu.h
>>> @@ -60,6 +60,9 @@ void cleanup_srcu_struct(struct srcu_struct *ssp);
>>> int __srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp);
>>> void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp);
>>> void synchronize_srcu(struct srcu_struct *ssp);
>>> +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp);
>>> +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp);
>>> +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie);
>>> #ifdef CONFIG_DEBUG_LOCK_ALLOC
>>> diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
>>> index d9edb67..c7f0c1f 100644
>>> --- a/include/linux/srcutiny.h
>>> +++ b/include/linux/srcutiny.h
>>> @@ -16,6 +16,7 @@
>>> struct srcu_struct {
>>> short srcu_lock_nesting[2]; /* srcu_read_lock() nesting depth. */
>>> unsigned short srcu_idx; /* Current reader array element in bit 0x2. */
>>> + unsigned short srcu_idx_max; /* Furthest future srcu_idx request. */
>>> u8 srcu_gp_running; /* GP workqueue running? */
>>> u8 srcu_gp_waiting; /* GP waiting for readers? */
>>> struct swait_queue_head srcu_wq;
>>> diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
>>> index 3bac1db..b073175 100644
>>> --- a/kernel/rcu/srcutiny.c
>>> +++ b/kernel/rcu/srcutiny.c
>>> @@ -34,6 +34,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp)
>>> ssp->srcu_gp_running = false;
>>> ssp->srcu_gp_waiting = false;
>>> ssp->srcu_idx = 0;
>>> + ssp->srcu_idx_max = 0;
>>> INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
>>> INIT_LIST_HEAD(&ssp->srcu_work.entry);
>>> return 0;
>>
>> Minor: cleanup_srcu_struct() can probably have 2 new sanity checks?
>>
>> WARN_ON(ssp->srcu_idx != ssp->srcu_idx_max);
>> WARN_ON(ssp->srcu_idx & 1);
>
> Good point, added and under test.
>
>> Thanks
>> Neeraj
>>
>>> @@ -114,7 +115,7 @@ void srcu_drive_gp(struct work_struct *wp)
>>> struct srcu_struct *ssp;
>>> ssp = container_of(wp, struct srcu_struct, srcu_work);
>>> - if (ssp->srcu_gp_running || !READ_ONCE(ssp->srcu_cb_head))
>>> + if (ssp->srcu_gp_running || USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
>>> return; /* Already running or nothing to do. */
>>> /* Remove recently arrived callbacks and wait for readers. */
>>> @@ -147,13 +148,18 @@ void srcu_drive_gp(struct work_struct *wp)
>>> * straighten that out.
>>> */
>>> WRITE_ONCE(ssp->srcu_gp_running, false);
>>> - if (READ_ONCE(ssp->srcu_cb_head))
>>> + if (USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
>>> schedule_work(&ssp->srcu_work);
>>> }
>>> EXPORT_SYMBOL_GPL(srcu_drive_gp);
>>> static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
>>> {
>>> + unsigned short cookie;
>>> +
>>> + cookie = get_state_synchronize_srcu(ssp);
>>> + if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
>>> + WRITE_ONCE(ssp->srcu_idx_max, cookie);
>>
>> Minor: Maybe we can return in the else part of USHORT_CMP_LT check, to avoid
>> scheduling work?
>
> How about like this?

Looks good!


Thanks
Neeraj

>
> static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
> {
> unsigned short cookie;
>
> cookie = get_state_synchronize_srcu(ssp);
> if (USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx_max), cookie))
> return;
> WRITE_ONCE(ssp->srcu_idx_max, cookie);
> if (!READ_ONCE(ssp->srcu_gp_running)) {
> if (likely(srcu_init_done))
> schedule_work(&ssp->srcu_work);
> else if (list_empty(&ssp->srcu_work.entry))
> list_add(&ssp->srcu_work.entry, &srcu_boot_list);
> }
> }
>
> Testing this next. ;-)
>
> Thanx, Paul
>
>> Thanks
>> Neeraj
>>
>>> if (!READ_ONCE(ssp->srcu_gp_running)) {
>>> if (likely(srcu_init_done))
>>> schedule_work(&ssp->srcu_work);
>>> @@ -196,6 +202,48 @@ void synchronize_srcu(struct srcu_struct *ssp)
>>> }
>>> EXPORT_SYMBOL_GPL(synchronize_srcu);
>>> +/*
>>> + * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
>>> + */
>>> +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
>>> +{
>>> + unsigned long ret;
>>> +
>>> + barrier();
>>> + ret = (READ_ONCE(ssp->srcu_idx) + 3) & ~0x1;
>>> + barrier();
>>> + return ret & USHRT_MAX;
>>> +}
>>> +EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
>>> +
>>> +/*
>>> + * start_poll_synchronize_srcu - Provide cookie and start grace period
>>> + *
>>> + * The difference between this and get_state_synchronize_srcu() is that
>>> + * this function ensures that the poll_state_synchronize_srcu() will
>>> + * eventually return the value true.
>>> + */
>>> +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
>>> +{
>>> + unsigned long ret = get_state_synchronize_srcu(ssp);
>>> +
>>> + srcu_gp_start_if_needed(ssp);
>>> + return ret;
>>> +}
>>> +EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
>>> +
>>> +/*
>>> + * poll_state_synchronize_srcu - Has cookie's grace period ended?
>>> + */
>>> +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
>>> +{
>>> + bool ret = USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx), cookie);
>>> +
>>> + barrier();
>>> + return ret;
>>> +}
>>> +EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
>>> +
>>> /* Lockdep diagnostics. */
>>> void __init rcu_scheduler_starting(void)
>>> {
>>>
>>
>> --
>> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of
>> the Code Aurora Forum, hosted by The Linux Foundation

--
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
member of the Code Aurora Forum, hosted by The Linux Foundation

2020-11-24 19:36:21

by Paul E. McKenney

Subject: Re: [PATCH v2 tip/core/rcu 4/6] srcu: Provide polling interfaces for Tiny SRCU grace periods

On Tue, Nov 24, 2020 at 10:44:24AM +0530, Neeraj Upadhyay wrote:
>
>
> On 11/24/2020 2:42 AM, Paul E. McKenney wrote:
> > On Mon, Nov 23, 2020 at 10:13:13AM +0530, Neeraj Upadhyay wrote:
> > >
> > >
> > > On 11/21/2020 6:29 AM, [email protected] wrote:
> > > > From: "Paul E. McKenney" <[email protected]>
> > > >
> > > > There is a need for a polling interface for SRCU grace
> > > > periods, so this commit supplies get_state_synchronize_srcu(),
> > > > start_poll_synchronize_srcu(), and poll_state_synchronize_srcu() for this
> > > > purpose. The first can be used if future grace periods are inevitable
> > > > (perhaps due to a later call_srcu() invocation), the second if future
> > > > grace periods might not otherwise happen, and the third to check if a
> > > > grace period has elapsed since the corresponding call to either of the
> > > > first two.
> > > >
> > > > As with get_state_synchronize_rcu() and cond_synchronize_rcu(),
> > > > the return value from either get_state_synchronize_srcu() or
> > > > start_poll_synchronize_srcu() must be passed in to a later call to
> > > > poll_state_synchronize_srcu().
> > > >
> > > > Link: https://lore.kernel.org/rcu/[email protected]/
> > > > Reported-by: Kent Overstreet <[email protected]>
> > > > [ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ]
> > > > [ paulmck: Apply feedback from Neeraj Upadhyay. ]
> > > > Link: https://lore.kernel.org/lkml/20201117004017.GA7444@paulmck-ThinkPad-P72/
> > > > Signed-off-by: Paul E. McKenney <[email protected]>
> > > > ---
> > > > include/linux/rcupdate.h | 2 ++
> > > > include/linux/srcu.h | 3 +++
> > > > include/linux/srcutiny.h | 1 +
> > > > kernel/rcu/srcutiny.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++--
> > > > 4 files changed, 56 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > > > index de08264..e09c0d8 100644
> > > > --- a/include/linux/rcupdate.h
> > > > +++ b/include/linux/rcupdate.h
> > > > @@ -33,6 +33,8 @@
> > > > #define ULONG_CMP_GE(a, b) (ULONG_MAX / 2 >= (a) - (b))
> > > > #define ULONG_CMP_LT(a, b) (ULONG_MAX / 2 < (a) - (b))
> > > > #define ulong2long(a) (*(long *)(&(a)))
> > > > +#define USHORT_CMP_GE(a, b) (USHRT_MAX / 2 >= (unsigned short)((a) - (b)))
> > > > +#define USHORT_CMP_LT(a, b) (USHRT_MAX / 2 < (unsigned short)((a) - (b)))
> > > > /* Exported common interfaces */
> > > > void call_rcu(struct rcu_head *head, rcu_callback_t func);
> > > > diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> > > > index e432cc9..a0895bb 100644
> > > > --- a/include/linux/srcu.h
> > > > +++ b/include/linux/srcu.h
> > > > @@ -60,6 +60,9 @@ void cleanup_srcu_struct(struct srcu_struct *ssp);
> > > > int __srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp);
> > > > void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp);
> > > > void synchronize_srcu(struct srcu_struct *ssp);
> > > > +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp);
> > > > +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp);
> > > > +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie);
> > > > #ifdef CONFIG_DEBUG_LOCK_ALLOC
> > > > diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
> > > > index d9edb67..c7f0c1f 100644
> > > > --- a/include/linux/srcutiny.h
> > > > +++ b/include/linux/srcutiny.h
> > > > @@ -16,6 +16,7 @@
> > > > struct srcu_struct {
> > > > short srcu_lock_nesting[2]; /* srcu_read_lock() nesting depth. */
> > > > unsigned short srcu_idx; /* Current reader array element in bit 0x2. */
> > > > + unsigned short srcu_idx_max; /* Furthest future srcu_idx request. */
> > > > u8 srcu_gp_running; /* GP workqueue running? */
> > > > u8 srcu_gp_waiting; /* GP waiting for readers? */
> > > > struct swait_queue_head srcu_wq;
> > > > diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
> > > > index 3bac1db..b073175 100644
> > > > --- a/kernel/rcu/srcutiny.c
> > > > +++ b/kernel/rcu/srcutiny.c
> > > > @@ -34,6 +34,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp)
> > > > ssp->srcu_gp_running = false;
> > > > ssp->srcu_gp_waiting = false;
> > > > ssp->srcu_idx = 0;
> > > > + ssp->srcu_idx_max = 0;
> > > > INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
> > > > INIT_LIST_HEAD(&ssp->srcu_work.entry);
> > > > return 0;
> > >
> > > Minor: cleanup_srcu_struct() can probably have 2 new sanity checks?
> > >
> > > WARN_ON(ssp->srcu_idx != ssp->srcu_idx_max);
> > > WARN_ON(ssp->srcu_idx & 1);
> >
> > Good point, added and under test.
> >
> > > Thanks
> > > Neeraj
> > >
> > > > @@ -114,7 +115,7 @@ void srcu_drive_gp(struct work_struct *wp)
> > > > struct srcu_struct *ssp;
> > > > ssp = container_of(wp, struct srcu_struct, srcu_work);
> > > > - if (ssp->srcu_gp_running || !READ_ONCE(ssp->srcu_cb_head))
> > > > + if (ssp->srcu_gp_running || USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
> > > > return; /* Already running or nothing to do. */
> > > > /* Remove recently arrived callbacks and wait for readers. */
> > > > @@ -147,13 +148,18 @@ void srcu_drive_gp(struct work_struct *wp)
> > > > * straighten that out.
> > > > */
> > > > WRITE_ONCE(ssp->srcu_gp_running, false);
> > > > - if (READ_ONCE(ssp->srcu_cb_head))
> > > > + if (USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
> > > > schedule_work(&ssp->srcu_work);
> > > > }
> > > > EXPORT_SYMBOL_GPL(srcu_drive_gp);
> > > > static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
> > > > {
> > > > + unsigned short cookie;
> > > > +
> > > > + cookie = get_state_synchronize_srcu(ssp);
> > > > + if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
> > > > + WRITE_ONCE(ssp->srcu_idx_max, cookie);
> > >
> > > Minor: Maybe we can return in the else part of USHORT_CMP_LT check, to avoid
> > > scheduling work?
> >
> > How about like this?
>
> Looks good!

Are you willing to give an ack or reviewed-by for either patch?

Thanx, Paul

> Thanks
> Neeraj
>
> >
> > static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
> > {
> > unsigned short cookie;
> >
> > cookie = get_state_synchronize_srcu(ssp);
> > if (USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx_max), cookie))
> > return;
> > WRITE_ONCE(ssp->srcu_idx_max, cookie);
> > if (!READ_ONCE(ssp->srcu_gp_running)) {
> > if (likely(srcu_init_done))
> > schedule_work(&ssp->srcu_work);
> > else if (list_empty(&ssp->srcu_work.entry))
> > list_add(&ssp->srcu_work.entry, &srcu_boot_list);
> > }
> > }
> >
> > Testing this next. ;-)
> >
> > Thanx, Paul
> >
> > > Thanks
> > > Neeraj
> > >
> > > > if (!READ_ONCE(ssp->srcu_gp_running)) {
> > > > if (likely(srcu_init_done))
> > > > schedule_work(&ssp->srcu_work);
> > > > @@ -196,6 +202,48 @@ void synchronize_srcu(struct srcu_struct *ssp)
> > > > }
> > > > EXPORT_SYMBOL_GPL(synchronize_srcu);
> > > > +/*
> > > > + * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
> > > > + */
> > > > +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
> > > > +{
> > > > + unsigned long ret;
> > > > +
> > > > + barrier();
> > > > + ret = (READ_ONCE(ssp->srcu_idx) + 3) & ~0x1;
> > > > + barrier();
> > > > + return ret & USHRT_MAX;
> > > > +}
> > > > +EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
> > > > +
> > > > +/*
> > > > + * start_poll_synchronize_srcu - Provide cookie and start grace period
> > > > + *
> > > > + * The difference between this and get_state_synchronize_srcu() is that
> > > > + * this function ensures that the poll_state_synchronize_srcu() will
> > > > + * eventually return the value true.
> > > > + */
> > > > +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
> > > > +{
> > > > + unsigned long ret = get_state_synchronize_srcu(ssp);
> > > > +
> > > > + srcu_gp_start_if_needed(ssp);
> > > > + return ret;
> > > > +}
> > > > +EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
> > > > +
> > > > +/*
> > > > + * poll_state_synchronize_srcu - Has cookie's grace period ended?
> > > > + */
> > > > +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
> > > > +{
> > > > + bool ret = USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx), cookie);
> > > > +
> > > > + barrier();
> > > > + return ret;
> > > > +}
> > > > +EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
> > > > +
> > > > /* Lockdep diagnostics. */
> > > > void __init rcu_scheduler_starting(void)
> > > > {
> > > >
> > >
> > > --
> > > QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of
> > > the Code Aurora Forum, hosted by The Linux Foundation
>
> --
> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of
> the Code Aurora Forum, hosted by The Linux Foundation

2020-11-25 04:44:09

by Neeraj Upadhyay

Subject: Re: [PATCH v2 tip/core/rcu 4/6] srcu: Provide polling interfaces for Tiny SRCU grace periods

On 11/25/2020 1:00 AM, Paul E. McKenney wrote:
> On Tue, Nov 24, 2020 at 10:44:24AM +0530, Neeraj Upadhyay wrote:
>>
>>
>> On 11/24/2020 2:42 AM, Paul E. McKenney wrote:
>>> On Mon, Nov 23, 2020 at 10:13:13AM +0530, Neeraj Upadhyay wrote:
>>>>
>>>>
>>>> On 11/21/2020 6:29 AM, [email protected] wrote:
>>>>> From: "Paul E. McKenney" <[email protected]>
>>>>>
>>>>> There is a need for a polling interface for SRCU grace
>>>>> periods, so this commit supplies get_state_synchronize_srcu(),
>>>>> start_poll_synchronize_srcu(), and poll_state_synchronize_srcu() for this
>>>>> purpose. The first can be used if future grace periods are inevitable
>>>>> (perhaps due to a later call_srcu() invocation), the second if future
>>>>> grace periods might not otherwise happen, and the third to check if a
>>>>> grace period has elapsed since the corresponding call to either of the
>>>>> first two.
>>>>>
>>>>> As with get_state_synchronize_rcu() and cond_synchronize_rcu(),
>>>>> the return value from either get_state_synchronize_srcu() or
>>>>> start_poll_synchronize_srcu() must be passed in to a later call to
>>>>> poll_state_synchronize_srcu().
>>>>>
>>>>> Link: https://lore.kernel.org/rcu/[email protected]/
>>>>> Reported-by: Kent Overstreet <[email protected]>
>>>>> [ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ]
>>>>> [ paulmck: Apply feedback from Neeraj Upadhyay. ]
>>>>> Link: https://lore.kernel.org/lkml/20201117004017.GA7444@paulmck-ThinkPad-P72/
>>>>> Signed-off-by: Paul E. McKenney <[email protected]>
>>>>> ---
>>>>> include/linux/rcupdate.h | 2 ++
>>>>> include/linux/srcu.h | 3 +++
>>>>> include/linux/srcutiny.h | 1 +
>>>>> kernel/rcu/srcutiny.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++--
>>>>> 4 files changed, 56 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
>>>>> index de08264..e09c0d8 100644
>>>>> --- a/include/linux/rcupdate.h
>>>>> +++ b/include/linux/rcupdate.h
>>>>> @@ -33,6 +33,8 @@
>>>>> #define ULONG_CMP_GE(a, b) (ULONG_MAX / 2 >= (a) - (b))
>>>>> #define ULONG_CMP_LT(a, b) (ULONG_MAX / 2 < (a) - (b))
>>>>> #define ulong2long(a) (*(long *)(&(a)))
>>>>> +#define USHORT_CMP_GE(a, b) (USHRT_MAX / 2 >= (unsigned short)((a) - (b)))
>>>>> +#define USHORT_CMP_LT(a, b) (USHRT_MAX / 2 < (unsigned short)((a) - (b)))
>>>>> /* Exported common interfaces */
>>>>> void call_rcu(struct rcu_head *head, rcu_callback_t func);
>>>>> diff --git a/include/linux/srcu.h b/include/linux/srcu.h
>>>>> index e432cc9..a0895bb 100644
>>>>> --- a/include/linux/srcu.h
>>>>> +++ b/include/linux/srcu.h
>>>>> @@ -60,6 +60,9 @@ void cleanup_srcu_struct(struct srcu_struct *ssp);
>>>>> int __srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp);
>>>>> void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp);
>>>>> void synchronize_srcu(struct srcu_struct *ssp);
>>>>> +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp);
>>>>> +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp);
>>>>> +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie);
>>>>> #ifdef CONFIG_DEBUG_LOCK_ALLOC
>>>>> diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
>>>>> index d9edb67..c7f0c1f 100644
>>>>> --- a/include/linux/srcutiny.h
>>>>> +++ b/include/linux/srcutiny.h
>>>>> @@ -16,6 +16,7 @@
>>>>> struct srcu_struct {
>>>>> short srcu_lock_nesting[2]; /* srcu_read_lock() nesting depth. */
>>>>> unsigned short srcu_idx; /* Current reader array element in bit 0x2. */
>>>>> + unsigned short srcu_idx_max; /* Furthest future srcu_idx request. */
>>>>> u8 srcu_gp_running; /* GP workqueue running? */
>>>>> u8 srcu_gp_waiting; /* GP waiting for readers? */
>>>>> struct swait_queue_head srcu_wq;
>>>>> diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
>>>>> index 3bac1db..b073175 100644
>>>>> --- a/kernel/rcu/srcutiny.c
>>>>> +++ b/kernel/rcu/srcutiny.c
>>>>> @@ -34,6 +34,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp)
>>>>> ssp->srcu_gp_running = false;
>>>>> ssp->srcu_gp_waiting = false;
>>>>> ssp->srcu_idx = 0;
>>>>> + ssp->srcu_idx_max = 0;
>>>>> INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
>>>>> INIT_LIST_HEAD(&ssp->srcu_work.entry);
>>>>> return 0;
>>>>
>>>> Minor: cleanup_srcu_struct() can probably have 2 new sanity checks?
>>>>
>>>> WARN_ON(ssp->srcu_idx != ssp->srcu_idx_max);
>>>> WARN_ON(ssp->srcu_idx & 1);
>>>
>>> Good point, added and under test.
>>>
>>>> Thanks
>>>> Neeraj
>>>>
>>>>> @@ -114,7 +115,7 @@ void srcu_drive_gp(struct work_struct *wp)
>>>>> struct srcu_struct *ssp;
>>>>> ssp = container_of(wp, struct srcu_struct, srcu_work);
>>>>> - if (ssp->srcu_gp_running || !READ_ONCE(ssp->srcu_cb_head))
>>>>> + if (ssp->srcu_gp_running || USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
>>>>> return; /* Already running or nothing to do. */
>>>>> /* Remove recently arrived callbacks and wait for readers. */
>>>>> @@ -147,13 +148,18 @@ void srcu_drive_gp(struct work_struct *wp)
>>>>> * straighten that out.
>>>>> */
>>>>> WRITE_ONCE(ssp->srcu_gp_running, false);
>>>>> - if (READ_ONCE(ssp->srcu_cb_head))
>>>>> + if (USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
>>>>> schedule_work(&ssp->srcu_work);
>>>>> }
>>>>> EXPORT_SYMBOL_GPL(srcu_drive_gp);
>>>>> static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
>>>>> {
>>>>> + unsigned short cookie;
>>>>> +
>>>>> + cookie = get_state_synchronize_srcu(ssp);
>>>>> + if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
>>>>> + WRITE_ONCE(ssp->srcu_idx_max, cookie);
>>>>
>>>> Minor: Maybe we can return in the else part of USHORT_CMP_LT check, to avoid
>>>> scheduling work?
>>>
>>> How about like this?
>>
>> Looks good!
>
> Are you willing to give an ack or reviewed-by for either patch?
>
> Thanx, Paul
>

For the version in rcu -dev

Reviewed-by: Neeraj Upadhyay <[email protected]>

Thanks
Neeraj

>> Thanks
>> Neeraj
>>
>>>
>>> static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
>>> {
>>> unsigned short cookie;
>>>
>>> cookie = get_state_synchronize_srcu(ssp);
>>> if (USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx_max), cookie))
>>> return;
>>> WRITE_ONCE(ssp->srcu_idx_max, cookie);
>>> if (!READ_ONCE(ssp->srcu_gp_running)) {
>>> if (likely(srcu_init_done))
>>> schedule_work(&ssp->srcu_work);
>>> else if (list_empty(&ssp->srcu_work.entry))
>>> list_add(&ssp->srcu_work.entry, &srcu_boot_list);
>>> }
>>> }
>>>
>>> Testing this next. ;-)
>>>
>>> Thanx, Paul
>>>
>>>> Thanks
>>>> Neeraj
>>>>
>>>>> if (!READ_ONCE(ssp->srcu_gp_running)) {
>>>>> if (likely(srcu_init_done))
>>>>> schedule_work(&ssp->srcu_work);
>>>>> @@ -196,6 +202,48 @@ void synchronize_srcu(struct srcu_struct *ssp)
>>>>> }
>>>>> EXPORT_SYMBOL_GPL(synchronize_srcu);
>>>>> +/*
>>>>> + * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
>>>>> + */
>>>>> +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
>>>>> +{
>>>>> + unsigned long ret;
>>>>> +
>>>>> + barrier();
>>>>> + ret = (READ_ONCE(ssp->srcu_idx) + 3) & ~0x1;
>>>>> + barrier();
>>>>> + return ret & USHRT_MAX;
>>>>> +}
>>>>> +EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
>>>>> +
>>>>> +/*
>>>>> + * start_poll_synchronize_srcu - Provide cookie and start grace period
>>>>> + *
>>>>> + * The difference between this and get_state_synchronize_srcu() is that
>>>>> + * this function ensures that the poll_state_synchronize_srcu() will
>>>>> + * eventually return the value true.
>>>>> + */
>>>>> +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
>>>>> +{
>>>>> + unsigned long ret = get_state_synchronize_srcu(ssp);
>>>>> +
>>>>> + srcu_gp_start_if_needed(ssp);
>>>>> + return ret;
>>>>> +}
>>>>> +EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
>>>>> +
>>>>> +/*
>>>>> + * poll_state_synchronize_srcu - Has cookie's grace period ended?
>>>>> + */
>>>>> +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
>>>>> +{
>>>>> + bool ret = USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx), cookie);
>>>>> +
>>>>> + barrier();
>>>>> + return ret;
>>>>> +}
>>>>> +EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
>>>>> +
>>>>> /* Lockdep diagnostics. */
>>>>> void __init rcu_scheduler_starting(void)
>>>>> {
>>>>>
>>>>
>>>> --
>>>> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of
>>>> the Code Aurora Forum, hosted by The Linux Foundation
>>
>> --
>> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of
>> the Code Aurora Forum, hosted by The Linux Foundation

--
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
member of the Code Aurora Forum, hosted by The Linux Foundation