On Thu, 2019-04-04 at 16:43 +-0800, Ming Lei wrote: +AD4 Just like aio/io+AF8-uring, we need to grab 2 refcount for queuing one +AD4 request, one is for submission, another is for completion. +AD4 +AD4 If the request isn't queued from plug code path, the refcount grabbed +AD4 in generic+AF8-make+AF8-request() serves for submission. In theroy, this +AD4 refcount should have been released after the sumission(async run queue) +AD4 is done. blk+AF8-freeze+AF8-queue() works with blk+AF8-sync+AF8-queue() together +AD4 for avoiding race between cleanup queue and IO submission, given async +AD4 run queue activities are canceled because hctx-+AD4-run+AF8-work is scheduled with +AD4 the refcount held, so it is fine to not hold the refcount when +AD4 running the run queue work function for dispatch IO. +AD4 +AD4 However, if request is staggered into plug list, and finally queued +AD4 from plug code path, the refcount in submission side is actually missed. +AD4 And we may start to run queue after queue is removed because the queue's +AD4 kobject refcount isn't guaranteed to be grabbed in flushing plug list +AD4 context, then kernel oops is triggered, see the following race: +AD4 +AD4 blk+AF8-mq+AF8-flush+AF8-plug+AF8-list(): +AD4 blk+AF8-mq+AF8-sched+AF8-insert+AF8-requests() +AD4 insert requests to sw queue or scheduler queue +AD4 blk+AF8-mq+AF8-run+AF8-hw+AF8-queue +AD4 +AD4 Because of concurrent run queue, all requests inserted above may be +AD4 completed before calling the above blk+AF8-mq+AF8-run+AF8-hw+AF8-queue. Then queue can +AD4 be freed during the above blk+AF8-mq+AF8-run+AF8-hw+AF8-queue(). +AD4 +AD4 Fixes the issue by grab .q+AF8-usage+AF8-counter before calling +AD4 blk+AF8-mq+AF8-sched+AF8-insert+AF8-requests() in blk+AF8-mq+AF8-flush+AF8-plug+AF8-list(). This way is +AD4 safe because the queue is absolutely alive before inserting request. Reviewed-by: Bart Van Assche +ADw-bvanassche+AEA-acm.org+AD4-
