Re: [RFC] [PATCH] Potential bug fix for blk_tag_queue

Matthew Wilcox
2008-08-26T02:46:37Z

On Mon, Aug 25, 2008 at 06:03:01PM -0700, Abhijeet Joglekar wrote:
> The following patch fixes a potential bug in the blk_tag_queue structure.
> "busy" is used to keep track of outstanding tags, is declared as int,
> and updated inside queue lock. For host-wide shared tag map, this corrupts
> the value of busy, which hits BUG_ON during __blk_free_tags.

I believe you to be right.

> Recommend converting busy to atomic_t and using atomic_macros to access it.

ugh, more atomic ops. How about just getting rid of it? Patch
untested:

diff --git a/block/blk-tag.c b/block/blk-tag.c
index 32667be..ed5166f 100644
--- a/block/blk-tag.c
+++ b/block/blk-tag.c
@@ -38,7 +38,8 @@ static int __blk_free_tags(struct blk_queue_tag *bqt)

retval = atomic_dec_and_test(&bqt->refcnt);
if (retval) {
- BUG_ON(bqt->busy);
+ BUG_ON(find_first_bit(bqt->tag_map, bqt->max_depth) <
+ bqt->max_depth);

kfree(bqt->tag_index);
bqt->tag_index = NULL;
@@ -147,7 +148,6 @@ static struct blk_queue_tag *__blk_queue_init_tags(struct request_queue *q,
if (init_tag_map(q, tags, depth))
goto fail;

- tags->busy = 0;
atomic_set(&tags->refcnt, 1);
return tags;
fail:
@@ -313,7 +313,6 @@ void blk_queue_end_tag(struct request_queue *q, struct request *rq)
* unlock memory barrier semantics.
*/
clear_bit_unlock(tag, bqt->tag_map);
- bqt->busy--;
}
EXPORT_SYMBOL(blk_queue_end_tag);

@@ -368,7 +367,6 @@ int blk_queue_start_tag(struct request_queue *q, struct request *rq)
bqt->tag_index[tag] = rq;
blkdev_dequeue_request(rq);
list_add(&rq->queuelist, &q->tag_busy_list);
- bqt->busy++;
return 0;
}
EXPORT_SYMBOL(blk_queue_start_tag);
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index e61f22b..c66c664 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -274,7 +274,6 @@ enum blk_queue_state {
struct blk_queue_tag {
struct request **tag_index; /* map of busy tags */
unsigned long *tag_map; /* bit map of free/busy tags */
- int busy; /* current depth */
int max_depth; /* what we will send to device */
int real_max_depth; /* what the array can hold */
atomic_t refcnt; /* map can be shared */

--
Matthew Wilcox Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours. We can't possibly take such
a retrograde step."