Re: [PATCH bpf-next v1 08/22] rqspinlock: Protect pending bit owners from stalls

[Waiman Long <llong@redhat.com>]

On 1/7/25 8:59 AM, Kumar Kartikeya Dwivedi wrote:
> The pending bit is used to avoid queueing in case the lock is
> uncontended, and has demonstrated benefits for the 2 contender scenario,
> esp. on x86. In case the pending bit is acquired and we wait for the
> locked bit to disappear, we may get stuck due to the lock owner not
> making progress. Hence, this waiting loop must be protected with a
> timeout check.
>
> To perform a graceful recovery once we decide to abort our lock
> acquisition attempt in this case, we must unset the pending bit since we
> own it. All waiters undoing their changes and exiting gracefully allows
> the lock word to be restored to the unlocked state once all participants
> (owner, waiters) have been recovered, and the lock remains usable.
> Hence, set the pending bit back to zero before returning to the caller.
>
> Introduce a lockevent (rqspinlock_lock_timeout) to capture timeout
> event statistics.
>
> Reviewed-by: Barret Rhoden <brho@google.com>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> ---
>   include/asm-generic/rqspinlock.h  |  2 +-
>   kernel/locking/lock_events_list.h |  5 +++++
>   kernel/locking/rqspinlock.c       | 28 +++++++++++++++++++++++-----
>   3 files changed, 29 insertions(+), 6 deletions(-)
>
> diff --git a/include/asm-generic/rqspinlock.h b/include/asm-generic/rqspinlock.h
> index 8ed266f4e70b..5c996a82e75f 100644
> --- a/include/asm-generic/rqspinlock.h
> +++ b/include/asm-generic/rqspinlock.h
> @@ -19,6 +19,6 @@ struct qspinlock;
>    */
>   #define RES_DEF_TIMEOUT (NSEC_PER_SEC / 2)
>   
> -extern void resilient_queued_spin_lock_slowpath(struct qspinlock *lock, u32 val, u64 timeout);
> +extern int resilient_queued_spin_lock_slowpath(struct qspinlock *lock, u32 val, u64 timeout);
>   
>   #endif /* __ASM_GENERIC_RQSPINLOCK_H */
> diff --git a/kernel/locking/lock_events_list.h b/kernel/locking/lock_events_list.h
> index 97fb6f3f840a..c5286249994d 100644
> --- a/kernel/locking/lock_events_list.h
> +++ b/kernel/locking/lock_events_list.h
> @@ -49,6 +49,11 @@ LOCK_EVENT(lock_use_node4)	/* # of locking ops that use 4th percpu node */
>   LOCK_EVENT(lock_no_node)	/* # of locking ops w/o using percpu node    */
>   #endif /* CONFIG_QUEUED_SPINLOCKS */
>   
> +/*
> + * Locking events for Resilient Queued Spin Lock
> + */
> +LOCK_EVENT(rqspinlock_lock_timeout)	/* # of locking ops that timeout	*/
> +
>   /*
>    * Locking events for rwsem
>    */

Since the build of rqspinlock.c is conditional on 
CONFIG_QUEUED_SPINLOCKS, this lock event should be inside the 
CONFIG_QUEUED_SPINLOCKS block.

Cheers,
Longman