raid5: fix a race of stripe count check

I hit another BUG_ON with e240c1839d11152b0355442. In __get_priority_stripe(),
stripe count equals to 0 initially. Between atomic_inc and BUG_ON,
get_active_stripe() finds the stripe. So the stripe count isn't 1 any more.

V2: keeps the BUG_ON suggested by Neil.

Signed-off-by: Shaohua Li <shli@fusionio.com>
Signed-off-by: NeilBrown <neilb@suse.de>

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 25247a8529124f7cc93eda0db7033655d52f2ef7..ad1b9bea446ebdbaea0083cb976ed4207c618cc1 100644 (file)
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -4370,8 +4370,7 @@ static struct stripe_head *__get_priority_stripe(struct r5conf *conf, int group)
                sh->group = NULL;
        }
        list_del_init(&sh->lru);
-       atomic_inc(&sh->count);
-       BUG_ON(atomic_read(&sh->count) != 1);
+       BUG_ON(atomic_inc_return(&sh->count) != 1);
        return sh;
 }