unimore/kernel-hacking-2024-linux-steffo
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bcachefs: Fix lost wake up

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If the reader acquires the read lock and then the writer enters the slow
path, while the reader proceeds to the unlock path, the following scenario
can occur without the change:

writer: pcpu_read_count(lock) return 1 (so __do_six_trylock will return 0)
reader: this_cpu_dec(*lock->readers)
reader: smp_mb()
reader: state = atomic_read(&lock->state) (there is no waiting flag set)
writer: six_set_bitmask()

then the writer will sleep forever.

Signed-off-by: Alan Huang <mmpgouride@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
This commit is contained in:
Alan Huang 2024-08-27 23:14:48 +08:00 committed by Kent Overstreet
parent d50d7a5fa4
commit e057a290ef
1 changed files with 9 additions and 3 deletions
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12
fs/bcachefs/six.c
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@ -169,11 +169,17 @@ static int __do_six_trylock(struct six_lock *lock, enum six_lock_type type,
ret = -1 - SIX_LOCK_write;
}
} else if (type == SIX_LOCK_write && lock->readers) {
if (try) {
if (try)
atomic_add(SIX_LOCK_HELD_write, &lock->state);
smp_mb__after_atomic();
}
/*
* Make sure atomic_add happens before pcpu_read_count and
* six_set_bitmask in slow path happens before pcpu_read_count.
*
* Paired with the smp_mb() in read lock fast path (per-cpu mode)
* and the one before atomic_read in read unlock path.
*/
smp_mb();
ret = !pcpu_read_count(lock);
if (try && !ret) {