Commit e057a290 authored by Alan Huang's avatar Alan Huang Committed by Kent Overstreet
Browse files

bcachefs: Fix lost wake up 



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: default avatarAlan Huang <mmpgouride@gmail.com>
Signed-off-by: default avatarKent Overstreet <kent.overstreet@linux.dev>
parent d50d7a5f

fs/bcachefs/six.c

+9 −3
Original line number Diff line number Diff line
@@ -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) {