There is a deadlock scenario between lockdep and rcu when
rcu nocb feature is enabled, just as following call stack:
rcuop/x
-000|queued_spin_lock_slowpath(lock = 0xFFFFFF817F2A8A80, val = ?)
-001|queued_spin_lock(inline) // try to hold nocb_gp_lock
-001|do_raw_spin_lock(lock = 0xFFFFFF817F2A8A80)
-002|__raw_spin_lock_irqsave(inline)
-002|_raw_spin_lock_irqsave(lock = 0xFFFFFF817F2A8A80)
-003|wake_nocb_gp_defer(inline)
-003|__call_rcu_nocb_wake(rdp = 0xFFFFFF817F30B680)
-004|__call_rcu_common(inline)
-004|call_rcu(head = 0xFFFFFFC082EECC28, func = ?)
-005|call_rcu_zapped(inline)
-005|free_zapped_rcu(ch = ?)// hold graph lock
-006|rcu_do_batch(rdp = 0xFFFFFF817F245680)
-007|nocb_cb_wait(inline)
-007|rcu_nocb_cb_kthread(arg = 0xFFFFFF817F245680)
-008|kthread(_create = 0xFFFFFF80803122C0)
-009|ret_from_fork(asm)
rcuop/y
-000|queued_spin_lock_slowpath(lock = 0xFFFFFFC08291BBC8, val = 0)
-001|queued_spin_lock()
-001|lockdep_lock()
-001|graph_lock() // try to hold graph lock
-002|lookup_chain_cache_add()
-002|validate_chain()
-003|lock_acquire
-004|_raw_spin_lock_irqsave(lock = 0xFFFFFF817F211D80)
-005|lock_timer_base(inline)
-006|mod_timer(inline)
-006|wake_nocb_gp_defer(inline)// hold nocb_gp_lock
-006|__call_rcu_nocb_wake(rdp = 0xFFFFFF817F2A8680)
-007|__call_rcu_common(inline)
-007|call_rcu(head = 0xFFFFFFC0822E0B58, func = ?)
-008|call_rcu_hurry(inline)
-008|rcu_sync_call(inline)
-008|rcu_sync_func(rhp = 0xFFFFFFC0822E0B58)
-009|rcu_do_batch(rdp = 0xFFFFFF817F266680)
-010|nocb_cb_wait(inline)
-010|rcu_nocb_cb_kthread(arg = 0xFFFFFF817F266680)
-011|kthread(_create = 0xFFFFFF8080363740)
-012|ret_from_fork(asm)
rcuop/x and rcuop/y are rcu nocb threads with the same nocb gp thread.
This patch release the graph lock before lockdep call_rcu.
Signed-off-by: Zhiguo Niu <[email protected]>
Signed-off-by: Xuewen Yan <[email protected]>
---
kernel/locking/lockdep.c | 38 +++++++++++++++++++++++++-------------
1 file changed, 25 insertions(+), 13 deletions(-)
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 151bd3d..c1d432a 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -6186,23 +6186,29 @@ static struct pending_free *get_pending_free(void)
/*
* Schedule an RCU callback if no RCU callback is pending. Must be called with
* the graph lock held.
+ *
+ * Return true if graph lock need be released by the caller, otherwise false
+ * means graph lock is released by itself.
*/
-static void call_rcu_zapped(struct pending_free *pf)
+static bool call_rcu_zapped(struct pending_free *pf)
{
WARN_ON_ONCE(inside_selftest());
if (list_empty(&pf->zapped))
- return;
+ return true;
if (delayed_free.scheduled)
- return;
+ return true;
delayed_free.scheduled = true;
WARN_ON_ONCE(delayed_free.pf + delayed_free.index != pf);
delayed_free.index ^= 1;
+ lockdep_unlock();
call_rcu(&delayed_free.rcu_head, free_zapped_rcu);
+
+ return false;
}
/* The caller must hold the graph lock. May be called from RCU context. */
@@ -6228,6 +6234,7 @@ static void free_zapped_rcu(struct rcu_head *ch)
{
struct pending_free *pf;
unsigned long flags;
+ bool need_unlock;
if (WARN_ON_ONCE(ch != &delayed_free.rcu_head))
return;
@@ -6243,9 +6250,9 @@ static void free_zapped_rcu(struct rcu_head *ch)
/*
* If there's anything on the open list, close and start a new callback.
*/
- call_rcu_zapped(delayed_free.pf + delayed_free.index);
-
- lockdep_unlock();
+ need_unlock = call_rcu_zapped(delayed_free.pf + delayed_free.index);
+ if (need_unlock)
+ lockdep_unlock();
raw_local_irq_restore(flags);
}
@@ -6286,6 +6293,7 @@ static void lockdep_free_key_range_reg(void *start, unsigned long size)
{
struct pending_free *pf;
unsigned long flags;
+ bool need_unlock;
init_data_structures_once();
@@ -6293,8 +6301,9 @@ static void lockdep_free_key_range_reg(void *start, unsigned long size)
lockdep_lock();
pf = get_pending_free();
__lockdep_free_key_range(pf, start, size);
- call_rcu_zapped(pf);
- lockdep_unlock();
+ need_unlock = call_rcu_zapped(pf);
+ if (need_unlock)
+ lockdep_unlock();
raw_local_irq_restore(flags);
/*
@@ -6390,6 +6399,7 @@ static void lockdep_reset_lock_reg(struct lockdep_map *lock)
struct pending_free *pf;
unsigned long flags;
int locked;
+ bool need_unlock;
raw_local_irq_save(flags);
locked = graph_lock();
@@ -6398,9 +6408,9 @@ static void lockdep_reset_lock_reg(struct lockdep_map *lock)
pf = get_pending_free();
__lockdep_reset_lock(pf, lock);
- call_rcu_zapped(pf);
-
- graph_unlock();
+ need_unlock = call_rcu_zapped(pf);
+ if (need_unlock)
+ graph_unlock();
out_irq:
raw_local_irq_restore(flags);
}
@@ -6446,6 +6456,7 @@ void lockdep_unregister_key(struct lock_class_key *key)
struct pending_free *pf;
unsigned long flags;
bool found = false;
+ bool need_unlock = true;
might_sleep();
@@ -6466,9 +6477,10 @@ void lockdep_unregister_key(struct lock_class_key *key)
if (found) {
pf = get_pending_free();
__lockdep_free_key_range(pf, key, 1);
- call_rcu_zapped(pf);
+ need_unlock = call_rcu_zapped(pf);
}
- lockdep_unlock();
+ if (need_unlock)
+ lockdep_unlock();
raw_local_irq_restore(flags);
/* Wait until is_dynamic_key() has finished accessing k->hash_entry. */
--
1.9.1
On Mon, Jan 15, 2024 at 04:53:16PM +0800, Zhiguo Niu wrote:
> There is a deadlock scenario between lockdep and rcu when
> rcu nocb feature is enabled, just as following call stack:
>
> rcuop/x
> -000|queued_spin_lock_slowpath(lock = 0xFFFFFF817F2A8A80, val = ?)
> -001|queued_spin_lock(inline) // try to hold nocb_gp_lock
> -001|do_raw_spin_lock(lock = 0xFFFFFF817F2A8A80)
> -002|__raw_spin_lock_irqsave(inline)
> -002|_raw_spin_lock_irqsave(lock = 0xFFFFFF817F2A8A80)
> -003|wake_nocb_gp_defer(inline)
> -003|__call_rcu_nocb_wake(rdp = 0xFFFFFF817F30B680)
> -004|__call_rcu_common(inline)
> -004|call_rcu(head = 0xFFFFFFC082EECC28, func = ?)
> -005|call_rcu_zapped(inline)
> -005|free_zapped_rcu(ch = ?)// hold graph lock
> -006|rcu_do_batch(rdp = 0xFFFFFF817F245680)
> -007|nocb_cb_wait(inline)
> -007|rcu_nocb_cb_kthread(arg = 0xFFFFFF817F245680)
> -008|kthread(_create = 0xFFFFFF80803122C0)
> -009|ret_from_fork(asm)
>
> rcuop/y
> -000|queued_spin_lock_slowpath(lock = 0xFFFFFFC08291BBC8, val = 0)
> -001|queued_spin_lock()
> -001|lockdep_lock()
> -001|graph_lock() // try to hold graph lock
> -002|lookup_chain_cache_add()
> -002|validate_chain()
> -003|lock_acquire
> -004|_raw_spin_lock_irqsave(lock = 0xFFFFFF817F211D80)
> -005|lock_timer_base(inline)
> -006|mod_timer(inline)
> -006|wake_nocb_gp_defer(inline)// hold nocb_gp_lock
> -006|__call_rcu_nocb_wake(rdp = 0xFFFFFF817F2A8680)
> -007|__call_rcu_common(inline)
> -007|call_rcu(head = 0xFFFFFFC0822E0B58, func = ?)
> -008|call_rcu_hurry(inline)
> -008|rcu_sync_call(inline)
> -008|rcu_sync_func(rhp = 0xFFFFFFC0822E0B58)
> -009|rcu_do_batch(rdp = 0xFFFFFF817F266680)
> -010|nocb_cb_wait(inline)
> -010|rcu_nocb_cb_kthread(arg = 0xFFFFFF817F266680)
> -011|kthread(_create = 0xFFFFFF8080363740)
> -012|ret_from_fork(asm)
>
> rcuop/x and rcuop/y are rcu nocb threads with the same nocb gp thread.
>
Nice! Looks like you find the root cause ;-) nocb_gp_lock and graph_lock
have an ABBA deadlock due to lockdep's dependency on RCU. I assume this
actually fixes the problem you saw?
However, I want to suggest a different fix, please see below:
> This patch release the graph lock before lockdep call_rcu.
>
> Signed-off-by: Zhiguo Niu <[email protected]>
> Signed-off-by: Xuewen Yan <[email protected]>
> ---
> kernel/locking/lockdep.c | 38 +++++++++++++++++++++++++-------------
> 1 file changed, 25 insertions(+), 13 deletions(-)
>
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 151bd3d..c1d432a 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -6186,23 +6186,29 @@ static struct pending_free *get_pending_free(void)
> /*
> * Schedule an RCU callback if no RCU callback is pending. Must be called with
> * the graph lock held.
> + *
> + * Return true if graph lock need be released by the caller, otherwise false
> + * means graph lock is released by itself.
> */
> -static void call_rcu_zapped(struct pending_free *pf)
> +static bool call_rcu_zapped(struct pending_free *pf)
> {
> WARN_ON_ONCE(inside_selftest());
>
> if (list_empty(&pf->zapped))
> - return;
> + return true;
>
> if (delayed_free.scheduled)
> - return;
> + return true;
>
> delayed_free.scheduled = true;
>
> WARN_ON_ONCE(delayed_free.pf + delayed_free.index != pf);
> delayed_free.index ^= 1;
>
> + lockdep_unlock();
> call_rcu(&delayed_free.rcu_head, free_zapped_rcu);
> +
> + return false;
> }
>
> /* The caller must hold the graph lock. May be called from RCU context. */
> @@ -6228,6 +6234,7 @@ static void free_zapped_rcu(struct rcu_head *ch)
> {
> struct pending_free *pf;
> unsigned long flags;
> + bool need_unlock;
>
> if (WARN_ON_ONCE(ch != &delayed_free.rcu_head))
> return;
> @@ -6243,9 +6250,9 @@ static void free_zapped_rcu(struct rcu_head *ch)
> /*
> * If there's anything on the open list, close and start a new callback.
> */
> - call_rcu_zapped(delayed_free.pf + delayed_free.index);
> -
> - lockdep_unlock();
> + need_unlock = call_rcu_zapped(delayed_free.pf + delayed_free.index);
> + if (need_unlock)
> + lockdep_unlock();
Instead of returning a bool to control the unlock, I think it's better
that we refactor the call_rcu_zapped() a bit, so it becomes a
prepare_call_rcu_zapped():
// See if we need to queue an RCU callback, must called with
// the lockdep lock held, returns false if either we don't have
// any pending free or the callback is already scheduled.
// Otherwise, a call_rcu() must follow this function call.
static bool prepare_call_rcu_zapped(struct pending_free *pf)
{
WARN_ON_ONCE(inside_selftest());
if (list_empty(&pf->zapped))
return false;
if (delayed_free.scheduled)
return false;
delayed_free.scheduled = true;
WARN_ON_ONCE(delayed_free.pf + delayed_free.index != pf);
delayed_free.index ^= 1;
return true;
}
, and here we can:
<lockdep_lock() is called previous>
need_callback = prepare_call_rcu_zapped(...);
lockdep_unlock();
raw_local_irq_restore(flags);
if (need_callback)
call_rcu(&delayed_free.rcu_head, free_zapped_rcu);
compared to your fix, we don't have a special logic where
call_rcu_zapped() can be an unlock in some conditions, which prevents
local correctness reasoning.
Thoughts?
Regards,
Boqun
> raw_local_irq_restore(flags);
> }
>
[...]
Hi Boqun
On Wed, Jan 17, 2024 at 1:47 AM Boqun Feng <[email protected]> wrote:
>
> On Mon, Jan 15, 2024 at 04:53:16PM +0800, Zhiguo Niu wrote:
> > There is a deadlock scenario between lockdep and rcu when
> > rcu nocb feature is enabled, just as following call stack:
> >
> > rcuop/x
> > -000|queued_spin_lock_slowpath(lock = 0xFFFFFF817F2A8A80, val = ?)
> > -001|queued_spin_lock(inline) // try to hold nocb_gp_lock
> > -001|do_raw_spin_lock(lock = 0xFFFFFF817F2A8A80)
> > -002|__raw_spin_lock_irqsave(inline)
> > -002|_raw_spin_lock_irqsave(lock = 0xFFFFFF817F2A8A80)
> > -003|wake_nocb_gp_defer(inline)
> > -003|__call_rcu_nocb_wake(rdp = 0xFFFFFF817F30B680)
> > -004|__call_rcu_common(inline)
> > -004|call_rcu(head = 0xFFFFFFC082EECC28, func = ?)
> > -005|call_rcu_zapped(inline)
> > -005|free_zapped_rcu(ch = ?)// hold graph lock
> > -006|rcu_do_batch(rdp = 0xFFFFFF817F245680)
> > -007|nocb_cb_wait(inline)
> > -007|rcu_nocb_cb_kthread(arg = 0xFFFFFF817F245680)
> > -008|kthread(_create = 0xFFFFFF80803122C0)
> > -009|ret_from_fork(asm)
> >
> > rcuop/y
> > -000|queued_spin_lock_slowpath(lock = 0xFFFFFFC08291BBC8, val = 0)
> > -001|queued_spin_lock()
> > -001|lockdep_lock()
> > -001|graph_lock() // try to hold graph lock
> > -002|lookup_chain_cache_add()
> > -002|validate_chain()
> > -003|lock_acquire
> > -004|_raw_spin_lock_irqsave(lock = 0xFFFFFF817F211D80)
> > -005|lock_timer_base(inline)
> > -006|mod_timer(inline)
> > -006|wake_nocb_gp_defer(inline)// hold nocb_gp_lock
> > -006|__call_rcu_nocb_wake(rdp = 0xFFFFFF817F2A8680)
> > -007|__call_rcu_common(inline)
> > -007|call_rcu(head = 0xFFFFFFC0822E0B58, func = ?)
> > -008|call_rcu_hurry(inline)
> > -008|rcu_sync_call(inline)
> > -008|rcu_sync_func(rhp = 0xFFFFFFC0822E0B58)
> > -009|rcu_do_batch(rdp = 0xFFFFFF817F266680)
> > -010|nocb_cb_wait(inline)
> > -010|rcu_nocb_cb_kthread(arg = 0xFFFFFF817F266680)
> > -011|kthread(_create = 0xFFFFFF8080363740)
> > -012|ret_from_fork(asm)
> >
> > rcuop/x and rcuop/y are rcu nocb threads with the same nocb gp thread.
> >
>
> Nice! Looks like you find the root cause ;-) nocb_gp_lock and graph_lock
> have an ABBA deadlock due to lockdep's dependency on RCU. I assume this
> actually fixes the problem you saw?
yes, this deadlock issue can be fixed by this fixes base our test.
>
> However, I want to suggest a different fix, please see below:
>
> > This patch release the graph lock before lockdep call_rcu.
> >
> > Signed-off-by: Zhiguo Niu <[email protected]>
> > Signed-off-by: Xuewen Yan <[email protected]>
> > ---
> > kernel/locking/lockdep.c | 38 +++++++++++++++++++++++++-------------
> > 1 file changed, 25 insertions(+), 13 deletions(-)
> >
> > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> > index 151bd3d..c1d432a 100644
> > --- a/kernel/locking/lockdep.c
> > +++ b/kernel/locking/lockdep.c
> > @@ -6186,23 +6186,29 @@ static struct pending_free *get_pending_free(void)
> > /*
> > * Schedule an RCU callback if no RCU callback is pending. Must be called with
> > * the graph lock held.
> > + *
> > + * Return true if graph lock need be released by the caller, otherwise false
> > + * means graph lock is released by itself.
> > */
> > -static void call_rcu_zapped(struct pending_free *pf)
> > +static bool call_rcu_zapped(struct pending_free *pf)
> > {
> > WARN_ON_ONCE(inside_selftest());
> >
> > if (list_empty(&pf->zapped))
> > - return;
> > + return true;
> >
> > if (delayed_free.scheduled)
> > - return;
> > + return true;
> >
> > delayed_free.scheduled = true;
> >
> > WARN_ON_ONCE(delayed_free.pf + delayed_free.index != pf);
> > delayed_free.index ^= 1;
> >
> > + lockdep_unlock();
> > call_rcu(&delayed_free.rcu_head, free_zapped_rcu);
> > +
> > + return false;
> > }
> >
> > /* The caller must hold the graph lock. May be called from RCU context */
> > @@ -6228,6 +6234,7 @@ static void free_zapped_rcu(struct rcu_head *ch)
> > {
> > struct pending_free *pf;
> > unsigned long flags;
> > + bool need_unlock;
> >
> > if (WARN_ON_ONCE(ch != &delayed_free.rcu_head))
> > return;
> > @@ -6243,9 +6250,9 @@ static void free_zapped_rcu(struct rcu_head *ch)
> > /*
> > * If there's anything on the open list, close and start a new callback.
> > */
> > - call_rcu_zapped(delayed_free.pf + delayed_free.index);
> > -
> > - lockdep_unlock();
> > + need_unlock = call_rcu_zapped(delayed_free.pf + delayed_free.index);
> > + if (need_unlock)
> > + lockdep_unlock();
>
> Instead of returning a bool to control the unlock, I think it's better
> that we refactor the call_rcu_zapped() a bit, so it becomes a
> prepare_call_rcu_zapped():
>
> // See if we need to queue an RCU callback, must called with
> // the lockdep lock held, returns false if either we don't have
> // any pending free or the callback is already scheduled.
> // Otherwise, a call_rcu() must follow this function call.
> static bool prepare_call_rcu_zapped(struct pending_free *pf)
> {
> WARN_ON_ONCE(inside_selftest());
>
> if (list_empty(&pf->zapped))
> return false;
>
> if (delayed_free.scheduled)
> return false;
>
> delayed_free.scheduled = true;
>
> WARN_ON_ONCE(delayed_free.pf + delayed_free.index != pf);
> delayed_free.index ^= 1;
>
> return true;
> }
>
> , and here we can:
>
> <lockdep_lock() is called previous>
> need_callback = prepare_call_rcu_zapped(...);
> lockdep_unlock();
> raw_local_irq_restore(flags);
>
> if (need_callback)
> call_rcu(&delayed_free.rcu_head, free_zapped_rcu);
>
> compared to your fix, we don't have a special logic where
> call_rcu_zapped() can be an unlock in some conditions, which prevents
> local correctness reasoning.
>
> Thoughts?
Thanks for your suggestions, It seems that your modification is more
reasonable.
I will modify PATCH v2 according to your suggestion.
Thanks!
>
> Regards,
> Boqun
>
> > raw_local_irq_restore(flags);
> > }
> >
> [...]
On Wed, Jan 17, 2024 at 1:47 AM Boqun Feng <[email protected]> wrote:
>
> On Mon, Jan 15, 2024 at 04:53:16PM +0800, Zhiguo Niu wrote:
> > There is a deadlock scenario between lockdep and rcu when
> > rcu nocb feature is enabled, just as following call stack:
> >
> > rcuop/x
> > -000|queued_spin_lock_slowpath(lock = 0xFFFFFF817F2A8A80, val = ?)
> > -001|queued_spin_lock(inline) // try to hold nocb_gp_lock
> > -001|do_raw_spin_lock(lock = 0xFFFFFF817F2A8A80)
> > -002|__raw_spin_lock_irqsave(inline)
> > -002|_raw_spin_lock_irqsave(lock = 0xFFFFFF817F2A8A80)
> > -003|wake_nocb_gp_defer(inline)
> > -003|__call_rcu_nocb_wake(rdp = 0xFFFFFF817F30B680)
> > -004|__call_rcu_common(inline)
> > -004|call_rcu(head = 0xFFFFFFC082EECC28, func = ?)
> > -005|call_rcu_zapped(inline)
> > -005|free_zapped_rcu(ch = ?)// hold graph lock
> > -006|rcu_do_batch(rdp = 0xFFFFFF817F245680)
> > -007|nocb_cb_wait(inline)
> > -007|rcu_nocb_cb_kthread(arg = 0xFFFFFF817F245680)
> > -008|kthread(_create = 0xFFFFFF80803122C0)
> > -009|ret_from_fork(asm)
> >
> > rcuop/y
> > -000|queued_spin_lock_slowpath(lock = 0xFFFFFFC08291BBC8, val = 0)
> > -001|queued_spin_lock()
> > -001|lockdep_lock()
> > -001|graph_lock() // try to hold graph lock
> > -002|lookup_chain_cache_add()
> > -002|validate_chain()
> > -003|lock_acquire
> > -004|_raw_spin_lock_irqsave(lock = 0xFFFFFF817F211D80)
> > -005|lock_timer_base(inline)
> > -006|mod_timer(inline)
> > -006|wake_nocb_gp_defer(inline)// hold nocb_gp_lock
> > -006|__call_rcu_nocb_wake(rdp = 0xFFFFFF817F2A8680)
> > -007|__call_rcu_common(inline)
> > -007|call_rcu(head = 0xFFFFFFC0822E0B58, func = ?)
> > -008|call_rcu_hurry(inline)
> > -008|rcu_sync_call(inline)
> > -008|rcu_sync_func(rhp = 0xFFFFFFC0822E0B58)
> > -009|rcu_do_batch(rdp = 0xFFFFFF817F266680)
> > -010|nocb_cb_wait(inline)
> > -010|rcu_nocb_cb_kthread(arg = 0xFFFFFF817F266680)
> > -011|kthread(_create = 0xFFFFFF8080363740)
> > -012|ret_from_fork(asm)
> >
> > rcuop/x and rcuop/y are rcu nocb threads with the same nocb gp thread.
> >
>
> Nice! Looks like you find the root cause ;-) nocb_gp_lock and graph_lock
> have an ABBA deadlock due to lockdep's dependency on RCU. I assume this
> actually fixes the problem you saw?
>
> However, I want to suggest a different fix, please see below:
>
> > This patch release the graph lock before lockdep call_rcu.
> >
> > Signed-off-by: Zhiguo Niu <[email protected]>
> > Signed-off-by: Xuewen Yan <[email protected]>
> > ---
> > kernel/locking/lockdep.c | 38 +++++++++++++++++++++++++-------------
> > 1 file changed, 25 insertions(+), 13 deletions(-)
> >
> > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> > index 151bd3d..c1d432a 100644
> > --- a/kernel/locking/lockdep.c
> > +++ b/kernel/locking/lockdep.c
> > @@ -6186,23 +6186,29 @@ static struct pending_free *get_pending_free(void)
> > /*
> > * Schedule an RCU callback if no RCU callback is pending. Must be called with
> > * the graph lock held.
> > + *
> > + * Return true if graph lock need be released by the caller, otherwise false
> > + * means graph lock is released by itself.
> > */
> > -static void call_rcu_zapped(struct pending_free *pf)
> > +static bool call_rcu_zapped(struct pending_free *pf)
> > {
> > WARN_ON_ONCE(inside_selftest());
> >
> > if (list_empty(&pf->zapped))
> > - return;
> > + return true;
> >
> > if (delayed_free.scheduled)
> > - return;
> > + return true;
> >
> > delayed_free.scheduled = true;
> >
> > WARN_ON_ONCE(delayed_free.pf + delayed_free.index != pf);
> > delayed_free.index ^= 1;
> >
> > + lockdep_unlock();
> > call_rcu(&delayed_free.rcu_head, free_zapped_rcu);
> > +
> > + return false;
> > }
> >
> > /* The caller must hold the graph lock. May be called from RCU context */
> > @@ -6228,6 +6234,7 @@ static void free_zapped_rcu(struct rcu_head *ch)
> > {
> > struct pending_free *pf;
> > unsigned long flags;
> > + bool need_unlock;
> >
> > if (WARN_ON_ONCE(ch != &delayed_free.rcu_head))
> > return;
> > @@ -6243,9 +6250,9 @@ static void free_zapped_rcu(struct rcu_head *ch)
> > /*
> > * If there's anything on the open list, close and start a new callback.
> > */
> > - call_rcu_zapped(delayed_free.pf + delayed_free.index);
> > -
> > - lockdep_unlock();
> > + need_unlock = call_rcu_zapped(delayed_free.pf + delayed_free.index);
> > + if (need_unlock)
> > + lockdep_unlock();
>
> Instead of returning a bool to control the unlock, I think it's better
> that we refactor the call_rcu_zapped() a bit, so it becomes a
> prepare_call_rcu_zapped():
>
> // See if we need to queue an RCU callback, must called with
> // the lockdep lock held, returns false if either we don't have
> // any pending free or the callback is already scheduled.
> // Otherwise, a call_rcu() must follow this function call.
> static bool prepare_call_rcu_zapped(struct pending_free *pf)
> {
> WARN_ON_ONCE(inside_selftest());
>
> if (list_empty(&pf->zapped))
> return false;
>
> if (delayed_free.scheduled)
> return false;
>
> delayed_free.scheduled = true;
>
> WARN_ON_ONCE(delayed_free.pf + delayed_free.index != pf);
> delayed_free.index ^= 1;
>
> return true;
> }
>
> , and here we can:
>
> <lockdep_lock() is called previous>
> need_callback = prepare_call_rcu_zapped(...);
> lockdep_unlock();
> raw_local_irq_restore(flags);
>
> if (need_callback)
> call_rcu(&delayed_free.rcu_head, free_zapped_rcu);
Would there be any problems if call_rcu is placed outside the shutdown
interrupt?
>
> compared to your fix, we don't have a special logic where
> call_rcu_zapped() can be an unlock in some conditions, which prevents
> local correctness reasoning.
>
> Thoughts?
>
> Regards,
> Boqun
>
> > raw_local_irq_restore(flags);
> > }
> >
> [...]
>
On 1/16/24 23:35, Xuewen Yan wrote:
> On Wed, Jan 17, 2024 at 1:47 AM Boqun Feng <[email protected]> wrote:
>> On Mon, Jan 15, 2024 at 04:53:16PM +0800, Zhiguo Niu wrote:
>>> There is a deadlock scenario between lockdep and rcu when
>>> rcu nocb feature is enabled, just as following call stack:
>>>
>>> rcuop/x
>>> -000|queued_spin_lock_slowpath(lock = 0xFFFFFF817F2A8A80, val = ?)
>>> -001|queued_spin_lock(inline) // try to hold nocb_gp_lock
>>> -001|do_raw_spin_lock(lock = 0xFFFFFF817F2A8A80)
>>> -002|__raw_spin_lock_irqsave(inline)
>>> -002|_raw_spin_lock_irqsave(lock = 0xFFFFFF817F2A8A80)
>>> -003|wake_nocb_gp_defer(inline)
>>> -003|__call_rcu_nocb_wake(rdp = 0xFFFFFF817F30B680)
>>> -004|__call_rcu_common(inline)
>>> -004|call_rcu(head = 0xFFFFFFC082EECC28, func = ?)
>>> -005|call_rcu_zapped(inline)
>>> -005|free_zapped_rcu(ch = ?)// hold graph lock
>>> -006|rcu_do_batch(rdp = 0xFFFFFF817F245680)
>>> -007|nocb_cb_wait(inline)
>>> -007|rcu_nocb_cb_kthread(arg = 0xFFFFFF817F245680)
>>> -008|kthread(_create = 0xFFFFFF80803122C0)
>>> -009|ret_from_fork(asm)
>>>
>>> rcuop/y
>>> -000|queued_spin_lock_slowpath(lock = 0xFFFFFFC08291BBC8, val = 0)
>>> -001|queued_spin_lock()
>>> -001|lockdep_lock()
>>> -001|graph_lock() // try to hold graph lock
>>> -002|lookup_chain_cache_add()
>>> -002|validate_chain()
>>> -003|lock_acquire
>>> -004|_raw_spin_lock_irqsave(lock = 0xFFFFFF817F211D80)
>>> -005|lock_timer_base(inline)
>>> -006|mod_timer(inline)
>>> -006|wake_nocb_gp_defer(inline)// hold nocb_gp_lock
>>> -006|__call_rcu_nocb_wake(rdp = 0xFFFFFF817F2A8680)
>>> -007|__call_rcu_common(inline)
>>> -007|call_rcu(head = 0xFFFFFFC0822E0B58, func = ?)
>>> -008|call_rcu_hurry(inline)
>>> -008|rcu_sync_call(inline)
>>> -008|rcu_sync_func(rhp = 0xFFFFFFC0822E0B58)
>>> -009|rcu_do_batch(rdp = 0xFFFFFF817F266680)
>>> -010|nocb_cb_wait(inline)
>>> -010|rcu_nocb_cb_kthread(arg = 0xFFFFFF817F266680)
>>> -011|kthread(_create = 0xFFFFFF8080363740)
>>> -012|ret_from_fork(asm)
>>>
>>> rcuop/x and rcuop/y are rcu nocb threads with the same nocb gp thread.
>>>
>> Nice! Looks like you find the root cause ;-) nocb_gp_lock and graph_lock
>> have an ABBA deadlock due to lockdep's dependency on RCU. I assume this
>> actually fixes the problem you saw?
>>
>> However, I want to suggest a different fix, please see below:
>>
>>> This patch release the graph lock before lockdep call_rcu.
>>>
>>> Signed-off-by: Zhiguo Niu <[email protected]>
>>> Signed-off-by: Xuewen Yan <[email protected]>
>>> ---
>>> kernel/locking/lockdep.c | 38 +++++++++++++++++++++++++-------------
>>> 1 file changed, 25 insertions(+), 13 deletions(-)
>>>
>>> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
>>> index 151bd3d..c1d432a 100644
>>> --- a/kernel/locking/lockdep.c
>>> +++ b/kernel/locking/lockdep.c
>>> @@ -6186,23 +6186,29 @@ static struct pending_free *get_pending_free(void)
>>> /*
>>> * Schedule an RCU callback if no RCU callback is pending. Must be called with
>>> * the graph lock held.
>>> + *
>>> + * Return true if graph lock need be released by the caller, otherwise false
>>> + * means graph lock is released by itself.
>>> */
>>> -static void call_rcu_zapped(struct pending_free *pf)
>>> +static bool call_rcu_zapped(struct pending_free *pf)
>>> {
>>> WARN_ON_ONCE(inside_selftest());
>>>
>>> if (list_empty(&pf->zapped))
>>> - return;
>>> + return true;
>>>
>>> if (delayed_free.scheduled)
>>> - return;
>>> + return true;
>>>
>>> delayed_free.scheduled = true;
>>>
>>> WARN_ON_ONCE(delayed_free.pf + delayed_free.index != pf);
>>> delayed_free.index ^= 1;
>>>
>>> + lockdep_unlock();
>>> call_rcu(&delayed_free.rcu_head, free_zapped_rcu);
>>> +
>>> + return false;
>>> }
>>>
>>> /* The caller must hold the graph lock. May be called from RCU context. */
>>> @@ -6228,6 +6234,7 @@ static void free_zapped_rcu(struct rcu_head *ch)
>>> {
>>> struct pending_free *pf;
>>> unsigned long flags;
>>> + bool need_unlock;
>>>
>>> if (WARN_ON_ONCE(ch != &delayed_free.rcu_head))
>>> return;
>>> @@ -6243,9 +6250,9 @@ static void free_zapped_rcu(struct rcu_head *ch)
>>> /*
>>> * If there's anything on the open list, close and start a new callback.
>>> */
>>> - call_rcu_zapped(delayed_free.pf + delayed_free.index);
>>> -
>>> - lockdep_unlock();
>>> + need_unlock = call_rcu_zapped(delayed_free.pf + delayed_free.index);
>>> + if (need_unlock)
>>> + lockdep_unlock();
>> Instead of returning a bool to control the unlock, I think it's better
>> that we refactor the call_rcu_zapped() a bit, so it becomes a
>> prepare_call_rcu_zapped():
>>
>> // See if we need to queue an RCU callback, must called with
>> // the lockdep lock held, returns false if either we don't have
>> // any pending free or the callback is already scheduled.
>> // Otherwise, a call_rcu() must follow this function call.
>> static bool prepare_call_rcu_zapped(struct pending_free *pf)
>> {
>> WARN_ON_ONCE(inside_selftest());
>>
>> if (list_empty(&pf->zapped))
>> return false;
>>
>> if (delayed_free.scheduled)
>> return false;
>>
>> delayed_free.scheduled = true;
>>
>> WARN_ON_ONCE(delayed_free.pf + delayed_free.index != pf);
>> delayed_free.index ^= 1;
>>
>> return true;
>> }
>>
>> , and here we can:
>>
>> <lockdep_lock() is called previous>
>> need_callback = prepare_call_rcu_zapped(...);
>> lockdep_unlock();
>> raw_local_irq_restore(flags);
>>
>> if (need_callback)
>> call_rcu(&delayed_free.rcu_head, free_zapped_rcu);
> Would there be any problems if call_rcu is placed outside the shutdown
> interrupt?
call_rcu() doesn't need to be called with interrupt disabled. In fact,
it calls local_irq_save() itself when necessary. So that is perfectly fine.
Cheers,
Longman