lkml.org 
[lkml]   [2017]   [Mar]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH v1 2/3] blk-mq: comment on races related with timeout handler
Date
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>
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 08a49c69738b..7068779d3bac 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -527,6 +527,15 @@ 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 sitation, 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;
@@ -700,6 +709,19 @@ 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
\
 
 \ /
  Last update: 2017-03-17 11:03    [W:0.078 / U:0.156 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site