blk-mq: comment on races related with timeout handler

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>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 7df9dbfab02245c303dc75e8cc55fb7378767e51..baebd6c8210e43bea01024c5b8a52510ad4a8067 100644 (file)
--- 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);