Re: [PATCH V8 1/7] blk-mq: grab .q_usage_counter when queuing request from plug code path

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On Sun, 2019-04-28 at 16:14 +-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.
+AD4 
+AD4 Cc: Dongli Zhang +ADw-dongli.zhang+AEA-oracle.com+AD4
+AD4 Cc: James Smart +ADw-james.smart+AEA-broadcom.com+AD4
+AD4 Cc: Bart Van Assche +ADw-bart.vanassche+AEA-wdc.com+AD4
+AD4 Cc: linux-scsi+AEA-vger.kernel.org,
+AD4 Cc: Martin K . Petersen +ADw-martin.petersen+AEA-oracle.com+AD4,
+AD4 Cc: Christoph Hellwig +ADw-hch+AEA-lst.de+AD4,
+AD4 Cc: James E . J . Bottomley +ADw-jejb+AEA-linux.vnet.ibm.com+AD4,
+AD4 Reviewed-by: Bart Van Assche +ADw-bvanassche+AEA-acm.org+AD4
+AD4 Reviewed-by: Johannes Thumshirn +ADw-jthumshirn+AEA-suse.de+AD4
+AD4 Reviewed-by: Hannes Reinecke +ADw-hare+AEA-suse.com+AD4
+AD4 Tested-by: James Smart +ADw-james.smart+AEA-broadcom.com+AD4
+AD4 Signed-off-by: Ming Lei +ADw-ming.lei+AEA-redhat.com+AD4

I added my +ACI-Reviewed-by+ACI to a previous version of this patch but not
to this version of this patch. Several +ACI-Reviewed-by+ACI tags probably
should be removed.

+AD4 diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
+AD4 index aa6bc5c02643..dfe83e7935d6 100644
+AD4 --- a/block/blk-mq-sched.c
+AD4 +-+-+- b/block/blk-mq-sched.c
+AD4 +AEAAQA -414,6 +-414,13 +AEAAQA void blk+AF8-mq+AF8-sched+AF8-insert+AF8-requests(struct blk+AF8-mq+AF8-hw+AF8-ctx +ACo-hctx,
+AD4  +AHs
+AD4         struct elevator+AF8-queue +ACo-e+ADs
+AD4  
+AD4 +-       /+ACo
+AD4 +-        +ACo blk+AF8-mq+AF8-sched+AF8-insert+AF8-requests() is called from flush plug
+AD4 +-        +ACo context only, and hold one usage counter to prevent queue
+AD4 +-        +ACo from being released.
+AD4 +-        +ACo-/
+AD4 +-       percpu+AF8-ref+AF8-get(+ACY-hctx-+AD4-queue-+AD4-q+AF8-usage+AF8-counter)+ADs
+AD4 +-
+AD4         e +AD0 hctx-+AD4-queue-+AD4-elevator+ADs
+AD4         if (e +ACYAJg e-+AD4-type-+AD4-ops.insert+AF8-requests)
+AD4                 e-+AD4-type-+AD4-ops.insert+AF8-requests(hctx, list, false)+ADs
+AD4 +AEAAQA -432,6 +-439,8 +AEAAQA void blk+AF8-mq+AF8-sched+AF8-insert+AF8-requests(struct blk+AF8-mq+AF8-hw+AF8-ctx +ACo-hctx,
+AD4         +AH0
+AD4  
+AD4         blk+AF8-mq+AF8-run+AF8-hw+AF8-queue(hctx, run+AF8-queue+AF8-async)+ADs
+AD4 +-
+AD4 +-       percpu+AF8-ref+AF8-put(+ACY-hctx-+AD4-queue-+AD4-q+AF8-usage+AF8-counter)+ADs
+AD4  +AH0

I think that 'hctx' can disappear if all requests queued by this function
finish just before blk+AF8-mq+AF8-run+AF8-hw+AF8-queue() returns and if the number of hardware
queues is changed from another thread. Shouldn't the request queue pointer be
stored in a local variable instead of reading hctx-+AD4-queue twice?

Bart.



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