Messages in this thread Patch in this message | | | From | Ross Lagerwall <> | Subject | [PATCH v2] blk-mq: Fix stall due to recursive flush plug | Date | Fri, 14 Jul 2023 11:11:06 +0100 |
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We have seen rare IO stalls as follows:
* blk_mq_plug_issue_direct() is entered with an mq_list containing two requests. * For the first request, it sets last == false and enters the driver's queue_rq callback. * The driver queue_rq callback indirectly calls schedule() which calls blk_flush_plug(). This may happen if the driver has the BLK_MQ_F_BLOCKING flag set and is allowed to sleep in ->queue_rq. * blk_flush_plug() handles the remaining request in the mq_list. mq_list is now empty. * The original call to queue_rq resumes (with last == false). * The loop in blk_mq_plug_issue_direct() terminates because there are no remaining requests in mq_list.
The IO is now stalled because the last request submitted to the driver had last == false and there was no subsequent call to commit_rqs().
Fix this by returning early in blk_mq_flush_plug_list() if rq_count is 0 which it will be in the recursive case, rather than checking if the mq_list is empty. At the same time, adjust one of the callers to skip the mq_list empty check as it is not necessary.
Fixes: dc5fc361d891 ("block: attempt direct issue of plug list") Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com> ---
In v2: * Update commit message and add a comment.
block/blk-core.c | 3 +-- block/blk-mq.c | 9 ++++++++- 2 files changed, 9 insertions(+), 3 deletions(-)
diff --git a/block/blk-core.c b/block/blk-core.c index 99d8b9812b18..90de50082146 100644 --- a/block/blk-core.c +++ b/block/blk-core.c @@ -1144,8 +1144,7 @@ void __blk_flush_plug(struct blk_plug *plug, bool from_schedule) { if (!list_empty(&plug->cb_list)) flush_plug_callbacks(plug, from_schedule); - if (!rq_list_empty(plug->mq_list)) - blk_mq_flush_plug_list(plug, from_schedule); + blk_mq_flush_plug_list(plug, from_schedule); /* * Unconditionally flush out cached requests, even if the unplug * event came from schedule. Since we know hold references to the diff --git a/block/blk-mq.c b/block/blk-mq.c index 5504719b970d..e6bd9c5f42bb 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -2742,7 +2742,14 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule) { struct request *rq; - if (rq_list_empty(plug->mq_list)) + /* + * We may have been called recursively midway through handling + * plug->mq_list via a schedule() in the driver's queue_rq() callback. + * To avoid mq_list changing under our feet, clear rq_count early and + * bail out specifically if rq_count is 0 rather than checking + * whether the mq_list is empty. + */ + if (plug->rq_count == 0) return; plug->rq_count = 0; -- 2.39.3
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