Re: [PATCH v2] blk-mq: Fix race between resetting the timer and completion handling
From: "tj@kernel.org" <tj@kernel.org>
Date: 2018-02-07 20:07:24
Subsystem:
block layer, the rest · Maintainers:
Jens Axboe, Linus Torvalds
Hello, Bart. On Wed, Feb 07, 2018 at 06:14:13PM +0000, Bart Van Assche wrote:
When I wrote my comment I was not sure whether or not non-reentrancy is guaranteed for work queue items. However, according to what I found in the workqueue implementation I think that is guaranteed. So it shouldn't be possible that the timer activated by blk_add_timer() gets handled before aborted_gstate is reset. But since the timeout handler and completion
Yeah, we're basically single threaded in the timeout path.
handlers can be executed by different CPUs, shouldn't a memory barrier be inserted between the blk_add_timer() call and resetting aborted_gsync to guarantee that a completion cannot occur before blk_add_timer() has reset RQF_MQ_TIMEOUT_EXPIRED?
Ah, you're right. u64_stat_sync doesn't imply barriers, so we want something like the following.
diff --git a/block/blk-mq.c b/block/blk-mq.c
index df93102..d6edf3b 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c@@ -593,7 +593,7 @@ static void blk_mq_rq_update_aborted_gstate(struct request *rq, u64 gstate) */ local_irq_save(flags); u64_stats_update_begin(&rq->aborted_gstate_sync); - rq->aborted_gstate = gstate; + smp_store_release(&rq->aborted_gstate, gstate); u64_stats_update_end(&rq->aborted_gstate_sync); local_irq_restore(flags); }
@@ -605,7 +605,7 @@ static u64 blk_mq_rq_aborted_gstate(struct request *rq) do { start = u64_stats_fetch_begin(&rq->aborted_gstate_sync); - aborted_gstate = rq->aborted_gstate; + aborted_gstate = smp_load_acquire(&rq->aborted_gstate); } while (u64_stats_fetch_retry(&rq->aborted_gstate_sync, start)); return aborted_gstate;
@@ -836,8 +836,8 @@ static void blk_mq_rq_timed_out(struct request *req, bool reserved) * ->aborted_gstate is set, this may lead to ignored * completions and further spurious timeouts. */ - blk_mq_rq_update_aborted_gstate(req, 0); blk_add_timer(req); + blk_mq_rq_update_aborted_gstate(req, 0); break; case BLK_EH_NOT_HANDLED: break;