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six locks: Fix a lost wakeup

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There was a lost wakeup between a read unlock in percpu mode and a write
lock. The unlock path unlocks, then executes a barrier, then checks for
waiters; correspondingly, the lock side should set the wait bit and
execute a barrier, then attempt to take the lock.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
This commit is contained in:
Kent Overstreet 2022-10-15 00:34:38 -04:00
parent 5b254da573
commit e4b7254c75
1 changed files with 8 additions and 3 deletions
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11
fs/bcachefs/six.c
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@ -198,6 +198,14 @@ retry:
atomic64_add(__SIX_VAL(write_locking, 1),
&lock->state.counter);
smp_mb__after_atomic();
} else if (!(lock->state.waiters & (1 << SIX_LOCK_write))) {
atomic64_add(__SIX_VAL(waiters, 1 << SIX_LOCK_write),
&lock->state.counter);
/*
* pairs with barrier after unlock and before checking
* for readers in unlock path
*/
smp_mb__after_atomic();
}
ret = !pcpu_read_count(lock);
@ -212,9 +220,6 @@ retry:
if (ret || try)
v -= __SIX_VAL(write_locking, 1);
if (!ret && !try && !(lock->state.waiters & (1 << SIX_LOCK_write)))
v += __SIX_VAL(waiters, 1 << SIX_LOCK_write);
if (try && !ret) {
old.v = atomic64_add_return(v, &lock->state.counter);
six_lock_wakeup(lock, old, SIX_LOCK_read);