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. ]
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 fed4a2d..e9bd6fb 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..b405811 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. */
@@ -147,14 +148,19 @@ 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;
+
if (!READ_ONCE(ssp->srcu_gp_running)) {
+ 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 (likely(srcu_init_done))
schedule_work(&ssp->srcu_work);
else if (list_empty(&ssp->srcu_work.entry))
@@ -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
Hi Paul,
On 11/17/2020 6:10 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. ]
> 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 fed4a2d..e9bd6fb 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..b405811 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. */
> @@ -147,14 +148,19 @@ 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)))
Should this be USHORT_CMP_LT ?
> 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;
> +
> if (!READ_ONCE(ssp->srcu_gp_running)) {
> + cookie = get_state_synchronize_srcu(ssp);
> + if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
> + WRITE_ONCE(ssp->srcu_idx_max, cookie);
I was thinking of a case which might break with this.
Consider a scenario, where GP is in progress and kworker is right
before below point, after executing callbacks:
void srcu_drive_gp(struct work_struct *wp) {
<snip>
while (lh) {
<cb execution loop>
}
>>> CURRENT EXECUTION POINT
WRITE_ONCE(ssp->srcu_gp_running, false);
if (USHORT_CMP_LT(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
schedule_work(&ssp->srcu_work);
}
Now, at this instance, srcu_gp_start_if_needed() runs and samples
srcu_gp_running and returns, without updating srcu_idx_max
static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
{
if (!READ_ONCE(ssp->srcu_gp_running)) returns true
<snip>
}
kworker running srcu_drive_gp() resumes and returns without queueing a
new schedule_work(&ssp->srcu_work); for new GP?
Prior to this patch, call_srcu() enqueues a cb before entering
srcu_gp_start_if_needed(), and srcu_drive_gp() observes this
queuing, and schedule a work for the new GP, for this scenario.
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);
So, should the "cookie" calculation and "srcu_idx_max" setting be moved
outside of ssp->srcu_gp_running check and maybe return the same cookie
to caller and use that as the returned cookie from
start_poll_synchronize_srcu() ?
srcu_gp_start_if_needed()
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)) {
<snip>
}
Thanks
Neeraj
> if (likely(srcu_init_done))
> schedule_work(&ssp->srcu_work);
> else if (list_empty(&ssp->srcu_work.entry))
> @@ -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
On Fri, Nov 20, 2020 at 05:28:32PM +0530, Neeraj Upadhyay wrote:
> Hi Paul,
>
> On 11/17/2020 6:10 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. ]
> > 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 fed4a2d..e9bd6fb 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..b405811 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. */
> > @@ -147,14 +148,19 @@ 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)))
>
> Should this be USHORT_CMP_LT ?
I believe that you are correct. As is, it works but does needless
grace periods.
> > 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;
> > +
> > if (!READ_ONCE(ssp->srcu_gp_running)) {
> > + cookie = get_state_synchronize_srcu(ssp);
> > + if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
> > + WRITE_ONCE(ssp->srcu_idx_max, cookie);
>
> I was thinking of a case which might break with this.
>
> Consider a scenario, where GP is in progress and kworker is right
> before below point, after executing callbacks:
>
> void srcu_drive_gp(struct work_struct *wp) {
> <snip>
We updated ->srcu_idx up here, correct? So it has bottom bit zero.
> while (lh) {
> <cb execution loop>
> }
> >>> CURRENT EXECUTION POINT
Keeping in mind that Tiny SRCU always runs !PREEMPT, this must be
due to an interrupt.
> WRITE_ONCE(ssp->srcu_gp_running, false);
>
> if (USHORT_CMP_LT(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
> schedule_work(&ssp->srcu_work);
> }
>
> Now, at this instance, srcu_gp_start_if_needed() runs and samples
> srcu_gp_running and returns, without updating srcu_idx_max
>
> static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
> {
> if (!READ_ONCE(ssp->srcu_gp_running)) returns true
> <snip>
> }
This could happen in an interrupt handler, so with you thus far.
> kworker running srcu_drive_gp() resumes and returns without queueing a new
> schedule_work(&ssp->srcu_work); for new GP?
>
> Prior to this patch, call_srcu() enqueues a cb before entering
> srcu_gp_start_if_needed(), and srcu_drive_gp() observes this
> queuing, and schedule a work for the new GP, for this scenario.
>
>
> 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);
>
> So, should the "cookie" calculation and "srcu_idx_max" setting be moved
> outside of ssp->srcu_gp_running check and maybe return the same cookie to
> caller and use that as the returned cookie from
> start_poll_synchronize_srcu() ?
>
> srcu_gp_start_if_needed()
> 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)) {
> <snip>
> }
I believe that you are quite correct, thank you!
But rcutorture does have a call_srcu() (really a ->call, but same if SRCU)
in a timer handler. The race window is quite narrow, so testing it might
be a challenge...
This is what I end up with:
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);
else if (list_empty(&ssp->srcu_work.entry))
list_add(&ssp->srcu_work.entry, &srcu_boot_list);
}
}
Does that look plausible?
Thanx, Paul
On 11/21/2020 5:43 AM, Paul E. McKenney wrote:
> On Fri, Nov 20, 2020 at 05:28:32PM +0530, Neeraj Upadhyay wrote:
>> Hi Paul,
>>
>> On 11/17/2020 6:10 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. ]
>>> 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 fed4a2d..e9bd6fb 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..b405811 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. */
>>> @@ -147,14 +148,19 @@ 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)))
>>
>> Should this be USHORT_CMP_LT ?
>
> I believe that you are correct. As is, it works but does needless
> grace periods.
>
>>> 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;
>>> +
>>> if (!READ_ONCE(ssp->srcu_gp_running)) {
>>> + cookie = get_state_synchronize_srcu(ssp);
>>> + if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
>>> + WRITE_ONCE(ssp->srcu_idx_max, cookie);
>>
>> I was thinking of a case which might break with this.
>>
>> Consider a scenario, where GP is in progress and kworker is right
>> before below point, after executing callbacks:
>>
>> void srcu_drive_gp(struct work_struct *wp) {
>> <snip>
>
> We updated ->srcu_idx up here, correct? So it has bottom bit zero.
>
>> while (lh) {
>> <cb execution loop>
>> }
>> >>> CURRENT EXECUTION POINT
>
> Keeping in mind that Tiny SRCU always runs !PREEMPT, this must be
> due to an interrupt.
>
Looking more, issue can happen, even when kworker is waiting for GP
completion @
swait_event_exclusive(ssp->srcu_wq,
!READ_ONCE(ssp->srcu_lock_nesting[idx]));
Other process can call call_srcu() and skip srcu_idx_max update, as
ssp->srcu_gp_running is true.
Thanks
Neeraj
>> WRITE_ONCE(ssp->srcu_gp_running, false);
>>
>> if (USHORT_CMP_LT(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
>> schedule_work(&ssp->srcu_work);
>> }
>>
>> Now, at this instance, srcu_gp_start_if_needed() runs and samples
>> srcu_gp_running and returns, without updating srcu_idx_max
>>
>> static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
>> {
>> if (!READ_ONCE(ssp->srcu_gp_running)) returns true
>> <snip>
>> }
>
> This could happen in an interrupt handler, so with you thus far.
>
>> kworker running srcu_drive_gp() resumes and returns without queueing a new
>> schedule_work(&ssp->srcu_work); for new GP?
>>
>> Prior to this patch, call_srcu() enqueues a cb before entering
>> srcu_gp_start_if_needed(), and srcu_drive_gp() observes this
>> queuing, and schedule a work for the new GP, for this scenario.
>>
>>
>> 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);
>>
>> So, should the "cookie" calculation and "srcu_idx_max" setting be moved
>> outside of ssp->srcu_gp_running check and maybe return the same cookie to
>> caller and use that as the returned cookie from
>> start_poll_synchronize_srcu() ?
>>
>> srcu_gp_start_if_needed()
>> 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)) {
>> <snip>
>> }
>
> I believe that you are quite correct, thank you!
>
> But rcutorture does have a call_srcu() (really a ->call, but same if SRCU)
> in a timer handler. The race window is quite narrow, so testing it might
> be a challenge...
>
> This is what I end up with:
>
> 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);
> else if (list_empty(&ssp->srcu_work.entry))
> list_add(&ssp->srcu_work.entry, &srcu_boot_list);
> }
> }
>
> Does that look plausible?
Looks good.
>
> Thanx, Paul
>
--
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
member of the Code Aurora Forum, hosted by The Linux Foundation
On Sun, Nov 22, 2020 at 07:57:26PM +0530, Neeraj Upadhyay wrote:
> On 11/21/2020 5:43 AM, Paul E. McKenney wrote:
> > On Fri, Nov 20, 2020 at 05:28:32PM +0530, Neeraj Upadhyay wrote:
> > > Hi Paul,
> > >
> > > On 11/17/2020 6:10 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. ]
> > > > 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 fed4a2d..e9bd6fb 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..b405811 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. */
> > > > @@ -147,14 +148,19 @@ 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)))
> > >
> > > Should this be USHORT_CMP_LT ?
> >
> > I believe that you are correct. As is, it works but does needless
> > grace periods.
> >
> > > > 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;
> > > > +
> > > > if (!READ_ONCE(ssp->srcu_gp_running)) {
> > > > + cookie = get_state_synchronize_srcu(ssp);
> > > > + if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
> > > > + WRITE_ONCE(ssp->srcu_idx_max, cookie);
> > >
> > > I was thinking of a case which might break with this.
> > >
> > > Consider a scenario, where GP is in progress and kworker is right
> > > before below point, after executing callbacks:
> > >
> > > void srcu_drive_gp(struct work_struct *wp) {
> > > <snip>
> >
> > We updated ->srcu_idx up here, correct? So it has bottom bit zero.
> >
> > > while (lh) {
> > > <cb execution loop>
> > > }
> > > >>> CURRENT EXECUTION POINT
> >
> > Keeping in mind that Tiny SRCU always runs !PREEMPT, this must be
> > due to an interrupt.
> >
> Looking more, issue can happen, even when kworker is waiting for GP
> completion @
>
> swait_event_exclusive(ssp->srcu_wq,
> !READ_ONCE(ssp->srcu_lock_nesting[idx]));
>
> Other process can call call_srcu() and skip srcu_idx_max update, as
> ssp->srcu_gp_running is true.
Good point! Does this mean that additional changes are required,
or does the fix below cover this situation as well?
Thanx, Paul
> Thanks
> Neeraj
>
> > > WRITE_ONCE(ssp->srcu_gp_running, false);
> > >
> > > if (USHORT_CMP_LT(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
> > > schedule_work(&ssp->srcu_work);
> > > }
> > >
> > > Now, at this instance, srcu_gp_start_if_needed() runs and samples
> > > srcu_gp_running and returns, without updating srcu_idx_max
> > >
> > > static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
> > > {
> > > if (!READ_ONCE(ssp->srcu_gp_running)) returns true
> > > <snip>
> > > }
> >
> > This could happen in an interrupt handler, so with you thus far.
> >
> > > kworker running srcu_drive_gp() resumes and returns without queueing a new
> > > schedule_work(&ssp->srcu_work); for new GP?
> > >
> > > Prior to this patch, call_srcu() enqueues a cb before entering
> > > srcu_gp_start_if_needed(), and srcu_drive_gp() observes this
> > > queuing, and schedule a work for the new GP, for this scenario.
> > >
> > >
> > > 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);
> > >
> > > So, should the "cookie" calculation and "srcu_idx_max" setting be moved
> > > outside of ssp->srcu_gp_running check and maybe return the same cookie to
> > > caller and use that as the returned cookie from
> > > start_poll_synchronize_srcu() ?
> > >
> > > srcu_gp_start_if_needed()
> > > 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)) {
> > > <snip>
> > > }
> >
> > I believe that you are quite correct, thank you!
> >
> > But rcutorture does have a call_srcu() (really a ->call, but same if SRCU)
> > in a timer handler. The race window is quite narrow, so testing it might
> > be a challenge...
> >
> > This is what I end up with:
> >
> > 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);
> > else if (list_empty(&ssp->srcu_work.entry))
> > list_add(&ssp->srcu_work.entry, &srcu_boot_list);
> > }
> > }
> >
> > Does that look plausible?
>
> Looks good.
>
> >
> > Thanx, Paul
> >
>
> --
> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of
> the Code Aurora Forum, hosted by The Linux Foundation
On 11/22/2020 11:31 PM, Paul E. McKenney wrote:
> On Sun, Nov 22, 2020 at 07:57:26PM +0530, Neeraj Upadhyay wrote:
>> On 11/21/2020 5:43 AM, Paul E. McKenney wrote:
>>> On Fri, Nov 20, 2020 at 05:28:32PM +0530, Neeraj Upadhyay wrote:
>>>> Hi Paul,
>>>>
>>>> On 11/17/2020 6:10 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. ]
>>>>> 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 fed4a2d..e9bd6fb 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..b405811 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. */
>>>>> @@ -147,14 +148,19 @@ 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)))
>>>>
>>>> Should this be USHORT_CMP_LT ?
>>>
>>> I believe that you are correct. As is, it works but does needless
>>> grace periods.
>>>
>>>>> 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;
>>>>> +
>>>>> if (!READ_ONCE(ssp->srcu_gp_running)) {
>>>>> + cookie = get_state_synchronize_srcu(ssp);
>>>>> + if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
>>>>> + WRITE_ONCE(ssp->srcu_idx_max, cookie);
>>>>
>>>> I was thinking of a case which might break with this.
>>>>
>>>> Consider a scenario, where GP is in progress and kworker is right
>>>> before below point, after executing callbacks:
>>>>
>>>> void srcu_drive_gp(struct work_struct *wp) {
>>>> <snip>
>>>
>>> We updated ->srcu_idx up here, correct? So it has bottom bit zero.
>>>
>>>> while (lh) {
>>>> <cb execution loop>
>>>> }
>>>> >>> CURRENT EXECUTION POINT
>>>
>>> Keeping in mind that Tiny SRCU always runs !PREEMPT, this must be
>>> due to an interrupt.
>>>
>> Looking more, issue can happen, even when kworker is waiting for GP
>> completion @
>>
>> swait_event_exclusive(ssp->srcu_wq,
>> !READ_ONCE(ssp->srcu_lock_nesting[idx]));
>>
>> Other process can call call_srcu() and skip srcu_idx_max update, as
>> ssp->srcu_gp_running is true.
>
> Good point! Does this mean that additional changes are required,
> or does the fix below cover this situation as well?
>
> Thanx, Paul
>
I think the current fix covers this. Just wanted to higlight that
the window is not small and a rcutorture test case might be able to
uncover the issue?
Thanks
Neeraj
>> Thanks
>> Neeraj
>>
>>>> WRITE_ONCE(ssp->srcu_gp_running, false);
>>>>
>>>> if (USHORT_CMP_LT(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
>>>> schedule_work(&ssp->srcu_work);
>>>> }
>>>>
>>>> Now, at this instance, srcu_gp_start_if_needed() runs and samples
>>>> srcu_gp_running and returns, without updating srcu_idx_max
>>>>
>>>> static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
>>>> {
>>>> if (!READ_ONCE(ssp->srcu_gp_running)) returns true
>>>> <snip>
>>>> }
>>>
>>> This could happen in an interrupt handler, so with you thus far.
>>>
>>>> kworker running srcu_drive_gp() resumes and returns without queueing a new
>>>> schedule_work(&ssp->srcu_work); for new GP?
>>>>
>>>> Prior to this patch, call_srcu() enqueues a cb before entering
>>>> srcu_gp_start_if_needed(), and srcu_drive_gp() observes this
>>>> queuing, and schedule a work for the new GP, for this scenario.
>>>>
>>>>
>>>> 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);
>>>>
>>>> So, should the "cookie" calculation and "srcu_idx_max" setting be moved
>>>> outside of ssp->srcu_gp_running check and maybe return the same cookie to
>>>> caller and use that as the returned cookie from
>>>> start_poll_synchronize_srcu() ?
>>>>
>>>> srcu_gp_start_if_needed()
>>>> 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)) {
>>>> <snip>
>>>> }
>>>
>>> I believe that you are quite correct, thank you!
>>>
>>> But rcutorture does have a call_srcu() (really a ->call, but same if SRCU)
>>> in a timer handler. The race window is quite narrow, so testing it might
>>> be a challenge...
>>>
>>> This is what I end up with:
>>>
>>> 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);
>>> else if (list_empty(&ssp->srcu_work.entry))
>>> list_add(&ssp->srcu_work.entry, &srcu_boot_list);
>>> }
>>> }
>>>
>>> Does that look plausible?
>>
>> Looks good.
>>
>>>
>>> Thanx, Paul
>>>
>>
>> --
>> 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
On Mon, Nov 23, 2020 at 10:04:23AM +0530, Neeraj Upadhyay wrote:
> On 11/22/2020 11:31 PM, Paul E. McKenney wrote:
> > On Sun, Nov 22, 2020 at 07:57:26PM +0530, Neeraj Upadhyay wrote:
> > > On 11/21/2020 5:43 AM, Paul E. McKenney wrote:
> > > > On Fri, Nov 20, 2020 at 05:28:32PM +0530, Neeraj Upadhyay wrote:
> > > > > Hi Paul,
> > > > >
> > > > > On 11/17/2020 6:10 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. ]
> > > > > > 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 fed4a2d..e9bd6fb 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..b405811 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. */
> > > > > > @@ -147,14 +148,19 @@ 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)))
> > > > >
> > > > > Should this be USHORT_CMP_LT ?
> > > >
> > > > I believe that you are correct. As is, it works but does needless
> > > > grace periods.
> > > >
> > > > > > 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;
> > > > > > +
> > > > > > if (!READ_ONCE(ssp->srcu_gp_running)) {
> > > > > > + cookie = get_state_synchronize_srcu(ssp);
> > > > > > + if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
> > > > > > + WRITE_ONCE(ssp->srcu_idx_max, cookie);
> > > > >
> > > > > I was thinking of a case which might break with this.
> > > > >
> > > > > Consider a scenario, where GP is in progress and kworker is right
> > > > > before below point, after executing callbacks:
> > > > >
> > > > > void srcu_drive_gp(struct work_struct *wp) {
> > > > > <snip>
> > > >
> > > > We updated ->srcu_idx up here, correct? So it has bottom bit zero.
> > > >
> > > > > while (lh) {
> > > > > <cb execution loop>
> > > > > }
> > > > > >>> CURRENT EXECUTION POINT
> > > >
> > > > Keeping in mind that Tiny SRCU always runs !PREEMPT, this must be
> > > > due to an interrupt.
> > > >
> > > Looking more, issue can happen, even when kworker is waiting for GP
> > > completion @
> > >
> > > swait_event_exclusive(ssp->srcu_wq,
> > > !READ_ONCE(ssp->srcu_lock_nesting[idx]));
> > >
> > > Other process can call call_srcu() and skip srcu_idx_max update, as
> > > ssp->srcu_gp_running is true.
> >
> > Good point! Does this mean that additional changes are required,
> > or does the fix below cover this situation as well?
>
> I think the current fix covers this. Just wanted to higlight that
> the window is not small and a rcutorture test case might be able to uncover
> the issue?
Thus far no luck, though. I am considering that this might be another
rcutorture bug. :-/
Thanx, Paul
> Thanks
> Neeraj
>
> > > Thanks
> > > Neeraj
> > >
> > > > > WRITE_ONCE(ssp->srcu_gp_running, false);
> > > > >
> > > > > if (USHORT_CMP_LT(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
> > > > > schedule_work(&ssp->srcu_work);
> > > > > }
> > > > >
> > > > > Now, at this instance, srcu_gp_start_if_needed() runs and samples
> > > > > srcu_gp_running and returns, without updating srcu_idx_max
> > > > >
> > > > > static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
> > > > > {
> > > > > if (!READ_ONCE(ssp->srcu_gp_running)) returns true
> > > > > <snip>
> > > > > }
> > > >
> > > > This could happen in an interrupt handler, so with you thus far.
> > > >
> > > > > kworker running srcu_drive_gp() resumes and returns without queueing a new
> > > > > schedule_work(&ssp->srcu_work); for new GP?
> > > > >
> > > > > Prior to this patch, call_srcu() enqueues a cb before entering
> > > > > srcu_gp_start_if_needed(), and srcu_drive_gp() observes this
> > > > > queuing, and schedule a work for the new GP, for this scenario.
> > > > >
> > > > >
> > > > > 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);
> > > > >
> > > > > So, should the "cookie" calculation and "srcu_idx_max" setting be moved
> > > > > outside of ssp->srcu_gp_running check and maybe return the same cookie to
> > > > > caller and use that as the returned cookie from
> > > > > start_poll_synchronize_srcu() ?
> > > > >
> > > > > srcu_gp_start_if_needed()
> > > > > 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)) {
> > > > > <snip>
> > > > > }
> > > >
> > > > I believe that you are quite correct, thank you!
> > > >
> > > > But rcutorture does have a call_srcu() (really a ->call, but same if SRCU)
> > > > in a timer handler. The race window is quite narrow, so testing it might
> > > > be a challenge...
> > > >
> > > > This is what I end up with:
> > > >
> > > > 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);
> > > > else if (list_empty(&ssp->srcu_work.entry))
> > > > list_add(&ssp->srcu_work.entry, &srcu_boot_list);
> > > > }
> > > > }
> > > >
> > > > Does that look plausible?
> > >
> > > Looks good.
> > >
> > > >
> > > > Thanx, Paul
> > > >
> > >
> > > --
> > > 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