[PATCH V2] blk-mq: fix race between complete and BLK_EH_RESET_TIMER

Ming Lei
2018-04-11T23:38:12Z

The normal request completion can be done before or during handling
BLK_EH_RESET_TIMER, and this race may cause the request to never
be completed since driver's .timeout() may always return
BLK_EH_RESET_TIMER.

This issue can't be fixed completely by driver, since the normal
completion can be done between returning .timeout() and handing
BLK_EH_RESET_TIMER.

This patch fixes this race by introducing rq state of MQ_RQ_COMPLETE_IN_RESET,
and reading/writing rq's state by holding queue lock, which can be
per-request actually, but just not necessary to introduce one lock for
so unusual event.

Cc: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Sagi Grimberg <sagi@grimberg.me>
Cc: Israel Rukshin <israelr@mellanox.com>,
Cc: Max Gurtovoy <maxg@mellanox.com>
Cc: stable@vger.kernel.org
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---

V2:
- rename the new flag as MQ_RQ_COMPLETE_IN_TIMEOUT
- fix lock uses in blk_mq_rq_timed_out
- document re-order between blk_add_timer() and
blk_mq_rq_update_aborted_gstate(req, 0)

block/blk-mq.c | 49 ++++++++++++++++++++++++++++++++++++++++++++-----
block/blk-mq.h | 1 +
2 files changed, 45 insertions(+), 5 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 0dc9e341c2a7..db1c84b2bb5e 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -630,10 +630,27 @@ void blk_mq_complete_request(struct request *rq)
* However, that would complicate paths which want to synchronize
* against us. Let stay in sync with the issue path so that
* hctx_lock() covers both issue and completion paths.
+ *
+ * Cover complete vs BLK_EH_RESET_TIMER race in slow path with
+ * helding queue lock.
*/
hctx_lock(hctx, &srcu_idx);
if (blk_mq_rq_aborted_gstate(rq) != rq->gstate)
__blk_mq_complete_request(rq);
+ else {
+ unsigned long flags;
+ bool need_complete = false;
+
+ spin_lock_irqsave(q->queue_lock, flags);
+ if (!blk_mq_rq_aborted_gstate(rq))
+ need_complete = true;
+ else
+ blk_mq_rq_update_state(rq, MQ_RQ_COMPLETE_IN_TIMEOUT);
+ spin_unlock_irqrestore(q->queue_lock, flags);
+
+ if (need_complete)
+ __blk_mq_complete_request(rq);
+ }
hctx_unlock(hctx, srcu_idx);
}
EXPORT_SYMBOL(blk_mq_complete_request);
@@ -814,6 +831,7 @@ static void blk_mq_rq_timed_out(struct request *req, bool reserved)
{
const struct blk_mq_ops *ops = req->q->mq_ops;
enum blk_eh_timer_return ret = BLK_EH_RESET_TIMER;
+ unsigned long flags;

req->rq_flags |= RQF_MQ_TIMEOUT_EXPIRED;

@@ -826,12 +844,33 @@ static void blk_mq_rq_timed_out(struct request *req, bool reserved)
break;
case BLK_EH_RESET_TIMER:
/*
- * As nothing prevents from completion happening while
- * ->aborted_gstate is set, this may lead to ignored
- * completions and further spurious timeouts.
+ * The normal completion may happen during handling the
+ * timeout, or even after returning from .timeout(), so
+ * once the request has been completed, we can't reset
+ * timer any more since this request may be handled as
+ * BLK_EH_RESET_TIMER in next timeout handling too, and
+ * it has to be completed in this situation.
+ *
+ * Holding the queue lock to cover read/write rq's
+ * aborted_gstate and normal state, so the race can be
+ * avoided completely.
+ *
+ * blk_add_timer() may be re-ordered with resetting
+ * aborted_gstate, and the only side-effec is that if this
+ * request is recycled after aborted_gstate is cleared, it
+ * may be timed out a bit late, that is what we can survive
+ * given timeout event is so unusual.
*/
- blk_mq_rq_update_aborted_gstate(req, 0);
- blk_add_timer(req);
+ spin_lock_irqsave(req->q->queue_lock, flags);
+ if (blk_mq_rq_state(req) != MQ_RQ_COMPLETE_IN_TIMEOUT) {
+ blk_add_timer(req);
+ blk_mq_rq_update_aborted_gstate(req, 0);
+ spin_unlock_irqrestore(req->q->queue_lock, flags);
+ } else {
+ blk_mq_rq_update_state(req, MQ_RQ_IN_FLIGHT);
+ spin_unlock_irqrestore(req->q->queue_lock, flags);
+ __blk_mq_complete_request(req);
+ }
break;
case BLK_EH_NOT_HANDLED:
break;
diff --git a/block/blk-mq.h b/block/blk-mq.h
index 88c558f71819..0426d048743d 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -35,6 +35,7 @@ enum mq_rq_state {
MQ_RQ_IDLE = 0,
MQ_RQ_IN_FLIGHT = 1,
MQ_RQ_COMPLETE = 2,
+ MQ_RQ_COMPLETE_IN_TIMEOUT = 3,

MQ_RQ_STATE_BITS = 2,
MQ_RQ_STATE_MASK = (1 << MQ_RQ_STATE_BITS) - 1,
--
2.9.5