Re: [PATCH] block: don't call blk_mq_run_hw_queues() for dead or dying queues

[Hannes Reinecke <hare@suse.de>]

On 3/26/19 2:43 PM, Bart Van Assche wrote:
> On 3/26/19 5:07 AM, Hannes Reinecke wrote:
>> When a queue is dying or dead there is no point in calling
>> blk_mq_run_hw_queues() in blk_mq_unquiesce_queue(); in fact, doing
>> so might crash the machine as the queue structures are in the
>> process of being deleted.
>>
>> Signed-off-by: Hannes Reinecke <hare@suse.com>
>> ---
>>   block/blk-mq.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>> index a9c181603cbd..b1eeba38bc79 100644
>> --- a/block/blk-mq.c
>> +++ b/block/blk-mq.c
>> @@ -258,7 +258,8 @@ void blk_mq_unquiesce_queue(struct request_queue *q)
>>       blk_queue_flag_clear(QUEUE_FLAG_QUIESCED, q);
>>       /* dispatch requests which are inserted during quiescing */
>> -    blk_mq_run_hw_queues(q, true);
>> +    if (!blk_queue_dying(q) && !blk_queue_dead(q))
>> +        blk_mq_run_hw_queues(q, true);
>>   }
>>   EXPORT_SYMBOL_GPL(blk_mq_unquiesce_queue);
> 
> Hi Hannes,
> 
> Please provide more context information. In the "dead" state the queue 
> must be run to make sure that all requests that were queued before the 
> "dead" state get processed. The blk_cleanup_queue() function is 
> responsible for stopping all code that can run the queue after all 
> requests have finished and before destruction of the data structures 
> needed for request processing starts.
> 
I have a crash with two processes competing for the same controller:

#0  0xffffffff983d3bcb in sbitmap_any_bit_set (sb=0xffff8a1b874ba0d8)
     at ../lib/sbitmap.c:181
#1  0xffffffff98366c05 in blk_mq_hctx_has_pending (hctx=0xffff8a1b874ba000)
     at ../block/blk-mq.c:66
#2  0xffffffff98366c85 in blk_mq_run_hw_queues (q=0xffff8a1b874ba0d8, 
async=true) at ../block/blk-mq.c:1292
#3  0xffffffff98366d3a in blk_mq_unquiesce_queue (q=<optimized out>)
     at ../block/blk-mq.c:265
#4  0xffffffffc01f3e0e in nvme_start_queues (ctrl=<optimized out>)
     at ../drivers/nvme/host/core.c:3658
#5  0xffffffffc01e843c in nvme_fc_delete_association 
(ctrl=0xffff8a1f9be5a000)
     at ../drivers/nvme/host/fc.c:2843
#6  0xffffffffc01e8544 in nvme_fc_delete_association (ctrl=<optimized out>)
     at ../drivers/nvme/host/fc.c:2918
#7  0xffffffffc01e8544 in __nvme_fc_terminate_io (ctrl=0xffff8a1f9be5a000)
     at ../drivers/nvme/host/fc.c:2911
#8  0xffffffffc01e8f09 in nvme_fc_reset_ctrl_work (work=0xffff8a1f9be5a6d0)
     at ../drivers/nvme/host/fc.c:2927
#9  0xffffffff980a224a in process_one_work (worker=0xffff8a1b73934f00, 
work=0xffff8a1f9be5a6d0) at ../kernel/workqueue.c:2092
#10 0xffffffff980a249b in worker_thread (__worker=0xffff8a1b73934f00)
     at ../kernel/workqueue.c:2226

#7  0xffffffff986d2e9a in wait_for_completion (x=0xffffa48eca88bc40)
     at ../kernel/sched/completion.c:125
#8  0xffffffff980f25ae in __synchronize_srcu (sp=0xffffffff9914fc20 
<debugfs_srcu>, do_norm=<optimized out>) at ../kernel/rcu/srcutree.c:851
#9  0xffffffff982d18b1 in debugfs_remove_recursive (dentry=<optimized out>)
     at ../fs/debugfs/inode.c:741
#10 0xffffffff98398ac5 in blk_mq_debugfs_unregister_hctx 
(hctx=0xffff8a1b7cccc000) at ../block/blk-mq-debugfs.c:897
#11 0xffffffff983661cf in blk_mq_exit_hctx (q=0xffff8a1f825e4040, 
set=0xffff8a1f9be5a0c0, hctx=0xffff8a1b7cccc000, hctx_idx=2) at 
../block/blk-mq.c:1987
#12 0xffffffff9836946a in blk_mq_exit_hw_queues (nr_queue=<optimized 
out>, set=<optimized out>, q=<optimized out>) at ../block/blk-mq.c:2017
#13 0xffffffff9836946a in blk_mq_free_queue (q=0xffff8a1f825e4040)
     at ../block/blk-mq.c:2506
#14 0xffffffff9835aac5 in blk_cleanup_queue (q=0xffff8a1f825e4040)
     at ../block/blk-core.c:691
#15 0xffffffffc01f5bc8 in nvme_ns_remove (ns=0xffff8a1f819e8f80)
     at ../drivers/nvme/host/core.c:3138
#16 0xffffffffc01f6fea in nvme_validate_ns (ctrl=0xffff8a1f9be5a308, nsid=5)
     at ../drivers/nvme/host/core.c:3164
#17 0xffffffffc01f9053 in nvme_scan_ns_list (nn=<optimized out>, 
ctrl=<optimized out>) at ../drivers/nvme/host/core.c:3202
#18 0xffffffffc01f9053 in nvme_scan_work (work=<optimized out>)
     at ../drivers/nvme/host/core.c:3280
#19 0xffffffff980a224a in process_one_work (worker=0xffff8a1b7349f6c0, 
work=0xffff8a1f9be5aba0) at ../kernel/workqueue.c:2092

Point is that the queue is already dead by the time nvme_start_queues() 
tries to flush existing requests (of which there are none, of course).
I had been looking into synchronizing scan_work and reset_work, but then 
I wasn't sure if that wouldn't deadlock somewhere.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
hare@suse.de			               +49 911 74053 688
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