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@xxxxxxxxxx>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
---
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
