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bcachefs: six locks: Fix lost wakeup

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In percpu reader mode, trylock() for read had a lost wakeup: on failure
to get the lock, we may have caused a writer to fail to get the lock,
because we temporarily elevated the reader count.

We need to check for waiters after decrementing the read count - not
before.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
This commit is contained in:
Kent Overstreet 2023-11-14 18:52:22 -05:00
parent 62d73dfc44
commit 61b85cb0d7
1 changed files with 5 additions and 2 deletions
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5
fs/bcachefs/six.c
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@ -163,8 +163,11 @@ static int __do_six_trylock(struct six_lock *lock, enum six_lock_type type,
this_cpu_sub(*lock->readers, !ret);
preempt_enable();
if (!ret && (old & SIX_LOCK_WAITING_write))
if (!ret) {
smp_mb();
if (atomic_read(&lock->state) & SIX_LOCK_WAITING_write)
ret = -1 - SIX_LOCK_write;
}
} else if (type == SIX_LOCK_write && lock->readers) {
if (try) {
atomic_add(SIX_LOCK_HELD_write, &lock->state);