Ensure that blk_mq_queue_enter() waits if mq_freeze_depth is not
zero. Ensure that the update of mq_freeze_depth by blk_mq_freeze_queue()
is visible by all CPU cores before that function waits on
mq_usage_counter.
It is unfortunate that this patch introduces an smp_mb() in the
hot path (blk_mq_queue_enter()) but I have not yet found a way to
avoid this.
I came across this code while analyzing a lockup triggered by
deleting a SCSI host created by the SRP initiator immediately
followed by a relogin.
Signed-off-by: Bart Van Assche <bart.vanass...@sandisk.com>
Cc: Christoph Hellwig <h...@lst.de>
Cc: Tejun Heo <t...@kernel.org>
Cc: <sta...@vger.kernel.org>
---
block/blk-mq.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 2077f0d..e3ad411 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -83,8 +83,13 @@ static int blk_mq_queue_enter(struct request_queue *q, gfp_t
gfp)
while (true) {
int ret;
- if (percpu_ref_tryget_live(&q->mq_usage_counter))
- return 0;
+ if (percpu_ref_tryget_live(&q->mq_usage_counter)) {
+ /* Order mq_use_counter and mq_freeze_depth accesses */
+ smp_mb();
+ if (!atomic_read(&q->mq_freeze_depth))
+ return 0;
+ percpu_ref_put(&q->mq_usage_counter);
+ }
if (!(gfp & __GFP_WAIT))
return -EBUSY;
@@ -136,6 +141,11 @@ static void blk_mq_freeze_queue_wait(struct request_queue
*q)
void blk_mq_freeze_queue(struct request_queue *q)
{
blk_mq_freeze_queue_start(q);
+ /*
+ * Ensure that the mq_freeze_depth update is visiable before
+ * mq_use_counter is read.
+ */
+ smp_mb();
blk_mq_freeze_queue_wait(q);
}
EXPORT_SYMBOL_GPL(blk_mq_freeze_queue);
--
2.1.4
--
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