From: Ming Lei <tom.leiming@gmail.com>
To: Jens Axboe <axboe@fb.com>,
linux-block@vger.kernel.org,
Christoph Hellwig <hch@infradead.org>
Cc: Bart Van Assche <bart.vanassche@sandisk.com>,
Hannes Reinecke <hare@suse.de>, Ming Lei <tom.leiming@gmail.com>
Subject: [PATCH v3 1/4] blk-mq: comment on races related with timeout handler
Date: Mon, 27 Mar 2017 20:06:55 +0800 [thread overview]
Message-ID: <20170327120658.29864-2-tom.leiming@gmail.com> (raw)
In-Reply-To: <20170327120658.29864-1-tom.leiming@gmail.com>
This patch adds comment on two races related with
timeout handler:
- requeue from queue busy vs. timeout
- rq free & reallocation vs. timeout
Both the races themselves and current solution aren't
explicit enough, so add comments on them.
Cc: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
---
block/blk-mq.c | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)
diff --git a/block/blk-mq.c b/block/blk-mq.c
index c212b9644a9f..b36f0481ba0e 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -523,6 +523,15 @@ void blk_mq_start_request(struct request *rq)
}
EXPORT_SYMBOL(blk_mq_start_request);
+/*
+ * When we reach here because queue is busy, REQ_ATOM_COMPLETE
+ * flag isn't set yet, so there may be race with timeout hanlder,
+ * but given rq->deadline is just set in .queue_rq() under
+ * this situation, the race won't be possible in reality because
+ * rq->timeout should be set as big enough to cover the window
+ * between blk_mq_start_request() called from .queue_rq() and
+ * clearing REQ_ATOM_STARTED here.
+ */
static void __blk_mq_requeue_request(struct request *rq)
{
struct request_queue *q = rq->q;
@@ -696,6 +705,19 @@ static void blk_mq_check_expired(struct blk_mq_hw_ctx *hctx,
if (!test_bit(REQ_ATOM_STARTED, &rq->atomic_flags))
return;
+ /*
+ * The rq being checked may have been freed and reallocated
+ * out already here, we avoid this race by checking rq->deadline
+ * and REQ_ATOM_COMPLETE flag together:
+ *
+ * - if rq->deadline is observed as new value because of
+ * reusing, the rq won't be timed out because of timing.
+ * - if rq->deadline is observed as previous value,
+ * REQ_ATOM_COMPLETE flag won't be cleared in reuse path
+ * because we put a barrier between setting rq->deadline
+ * and clearing the flag in blk_mq_start_request(), so
+ * this rq won't be timed out too.
+ */
if (time_after_eq(jiffies, rq->deadline)) {
if (!blk_mark_rq_complete(rq))
blk_mq_rq_timed_out(rq, reserved);
--
2.9.3
next prev parent reply other threads:[~2017-03-27 12:07 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-27 12:06 [PATCH v3 0/4] block: misc changes Ming Lei
2017-03-27 12:06 ` Ming Lei [this message]
2017-03-27 12:11 ` [PATCH v3 1/4] blk-mq: comment on races related with timeout handler Johannes Thumshirn
2017-03-27 12:06 ` [PATCH v3 2/4] block: add a read barrier in blk_queue_enter() Ming Lei
2017-03-27 12:14 ` Johannes Thumshirn
2017-03-27 15:44 ` Bart Van Assche
2017-03-27 12:06 ` [PATCH v3 3/4] block: rename blk_mq_freeze_queue_start() Ming Lei
2017-03-27 12:15 ` Johannes Thumshirn
2017-03-27 12:06 ` [PATCH v3 4/4] block: block new I/O just after queue is set as dying Ming Lei
2017-03-27 12:16 ` Johannes Thumshirn
2017-03-27 15:49 ` Bart Van Assche
2017-03-29 2:11 ` [PATCH v3 0/4] block: misc changes Ming Lei
2017-03-29 14:00 ` Jens Axboe
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