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futex: Decrease the waiter count before the unlock operation

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To support runtime resizing of the process private hash, it's required
to not use the obtained hash bucket once the reference count has been
dropped. The reference will be dropped after the unlock of the hash
bucket.
The amount of waiters is decremented after the unlock operation. There
is no requirement that this needs to happen after the unlock. The
increment happens before acquiring the lock to signal early that there
will be a waiter. The waiter can avoid blocking on the lock if it is
known that there will be no waiter.
There is no difference in terms of ordering if the decrement happens
before or after the unlock.

Decrease the waiter count before the unlock operation.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250416162921.513656-10-bigeasy@linutronix.de
This commit is contained in:
Sebastian Andrzej Siewior
2025-05-03 12:02:06 +02:00
committed by Peter Zijlstra
parent 3f6b233018
commit fe00e88d21
2 changed files with 5 additions and 5 deletions
Download Patch File Download Diff File Expand all files Collapse all files
kernel/futex/core.c
+1 -1
View File
@@ -537,8 +537,8 @@ void futex_q_lock(struct futex_q *q, struct futex_hash_bucket *hb)
void futex_q_unlock(struct futex_hash_bucket *hb)
__releases(&hb->lock)
{
spin_unlock(&hb->lock);
futex_hb_waiters_dec(hb);
spin_unlock(&hb->lock);
}
void __futex_queue(struct futex_q *q, struct futex_hash_bucket *hb,
kernel/futex/requeue.c
+4 -4
View File
@@ -456,8 +456,8 @@ retry_private:
ret = futex_get_value_locked(&curval, uaddr1);
if (unlikely(ret)) {
double_unlock_hb(hb1, hb2);
futex_hb_waiters_dec(hb2);
double_unlock_hb(hb1, hb2);
ret = get_user(curval, uaddr1);
if (ret)
@@ -542,8 +542,8 @@ retry_private:
* waiter::requeue_state is correct.
*/
case -EFAULT:
double_unlock_hb(hb1, hb2);
futex_hb_waiters_dec(hb2);
double_unlock_hb(hb1, hb2);
ret = fault_in_user_writeable(uaddr2);
if (!ret)
goto retry;
@@ -556,8 +556,8 @@ retry_private:
* exit to complete.
* - EAGAIN: The user space value changed.
*/
double_unlock_hb(hb1, hb2);
futex_hb_waiters_dec(hb2);
double_unlock_hb(hb1, hb2);
/*
* Handle the case where the owner is in the middle of
* exiting. Wait for the exit to complete otherwise
@@ -674,8 +674,8 @@ retry_private:
put_pi_state(pi_state);
out_unlock:
double_unlock_hb(hb1, hb2);
futex_hb_waiters_dec(hb2);
double_unlock_hb(hb1, hb2);
}
wake_up_q(&wake_q);
return ret ? ret : task_count;