Messages in this thread Patch in this message | | | From | Junxiao Bi <> | Subject | [PATCH] block: fix io hung by block throttle | Date | Wed, 14 Apr 2021 14:18:30 -0700 |
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There is a race bug which can cause io hung when multiple processes run parallel in rq_qos_wait(). Let assume there were 4 processes P1/P2/P3/P4, P1/P2 were at the entry of rq_qos_wait, and P3/P4 were waiting for io done, 2 io were inflight, the inflight io limit was 2. See race below.
void rq_qos_wait() { ... bool has_sleeper;
>>>> P3/P4 were in sleeper list, has_sleeper was true for both P1 and P2. has_sleeper = wq_has_sleeper(&rqw->wait); if (!has_sleeper && acquire_inflight_cb(rqw, private_data)) return;
>>>> 2 inflight io done, P3/P4 were waken up to issue 2 new io. >>>> 2 new io done, no inflight io.
>>>> P1/P2 were added to the sleeper list, 2 entry in the list prepare_to_wait_exclusive(&rqw->wait, &data.wq, TASK_UNINTERRUPTIBLE);
>>>> P1/P2 were in the sleeper list, has_sleeper was true for P1/P2. has_sleeper = !wq_has_single_sleeper(&rqw->wait); do { /* The memory barrier in set_task_state saves us here. */ if (data.got_token) break; if (!has_sleeper && acquire_inflight_cb(rqw, private_data)) { finish_wait(&rqw->wait, &data.wq);
/* * We raced with wbt_wake_function() getting a token, * which means we now have two. Put our local token * and wake anyone else potentially waiting for one. */ smp_rmb(); if (data.got_token) cleanup_cb(rqw, private_data); break; }
>>>> P1/P2 hung here forever. New io requests will also hung here. io_schedule(); has_sleeper = true; set_current_state(TASK_UNINTERRUPTIBLE); } while (1); finish_wait(&rqw->wait, &data.wq); }
Cc: stable@vger.kernel.org Signed-off-by: Junxiao Bi <junxiao.bi@oracle.com> --- block/blk-rq-qos.c | 9 +++------ 1 file changed, 3 insertions(+), 6 deletions(-)
diff --git a/block/blk-rq-qos.c b/block/blk-rq-qos.c index 656460636ad3..04d888c99bc0 100644 --- a/block/blk-rq-qos.c +++ b/block/blk-rq-qos.c @@ -260,19 +260,17 @@ void rq_qos_wait(struct rq_wait *rqw, void *private_data, .cb = acquire_inflight_cb, .private_data = private_data, }; - bool has_sleeper; - has_sleeper = wq_has_sleeper(&rqw->wait); - if (!has_sleeper && acquire_inflight_cb(rqw, private_data)) + if (!wq_has_sleeper(&rqw->wait) + && acquire_inflight_cb(rqw, private_data)) return; prepare_to_wait_exclusive(&rqw->wait, &data.wq, TASK_UNINTERRUPTIBLE); - has_sleeper = !wq_has_single_sleeper(&rqw->wait); do { /* The memory barrier in set_task_state saves us here. */ if (data.got_token) break; - if (!has_sleeper && acquire_inflight_cb(rqw, private_data)) { + if (acquire_inflight_cb(rqw, private_data)) { finish_wait(&rqw->wait, &data.wq); /* @@ -286,7 +284,6 @@ void rq_qos_wait(struct rq_wait *rqw, void *private_data, break; } io_schedule(); - has_sleeper = true; set_current_state(TASK_UNINTERRUPTIBLE); } while (1); finish_wait(&rqw->wait, &data.wq); -- 2.24.3 (Apple Git-128)
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