Re: [PATCH] Fix race with shared tag queue maps

Jens Axboe
2007-09-13T15:22:38Z

On Thu, Sep 13 2007, Linus Torvalds wrote:
>
>
> On Thu, 13 Sep 2007, Jens Axboe wrote:
> > +
> > + /*
> > + * Ensure ordering with tag section
> > + */
> > + smp_mb__before_clear_bit();
> > +
> > + if (unlikely(!test_and_clear_bit(tag, bqt->tag_map))) {
>
> You don't need the "smp_mb__before_clear_bit()" there.
>
> The regular "clear_bit()" needs it, but the "test_and_xxx()" versions are
> architecturally defined to be memory barriers, exactly because they are
> regularly used for locking.
>
> This is even documented - see Documentation/atomic_ops.txt.

My bad, I think I added the smp_mb__before_clear_bit() when it was
__test_and_set_bit() like in the first hunk.

diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index a15845c..dfc9f22 100644
--- a/block/ll_rw_blk.c
+++ b/block/ll_rw_blk.c
@@ -1075,12 +1075,6 @@ void blk_queue_end_tag(struct request_queue *q, struct request *rq)
*/
return;

- if (unlikely(!__test_and_clear_bit(tag, bqt->tag_map))) {
- printk(KERN_ERR "%s: attempt to clear non-busy tag (%d)\n",
- __FUNCTION__, tag);
- return;
- }
-
list_del_init(&rq->queuelist);
rq->cmd_flags &= ~REQ_QUEUED;
rq->tag = -1;
@@ -1090,6 +1084,18 @@ void blk_queue_end_tag(struct request_queue *q, struct request *rq)
__FUNCTION__, tag);

bqt->tag_index[tag] = NULL;
+
+ if (unlikely(!test_and_clear_bit(tag, bqt->tag_map))) {
+ printk(KERN_ERR "%s: attempt to clear non-busy tag (%d)\n",
+ __FUNCTION__, tag);
+ return;
+ }
+
+ /*
+ * Ensure ordering between ->tag_index[tag] clear and tag clear
+ */
+ smp_mb__after_clear_bit();
+
bqt->busy--;
}

--
Jens Axboe