bcachefs: six locks: Fix lost wakeup

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>

fs/bcachefs/six.c

@@ -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))
-			ret = -1 - SIX_LOCK_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);