[dm-devel] [PATCH] Fix deadlock with request based dm and some drivers

[Jens Axboe]

On 2012-11-06 13:14, Jun'ichi Nomura wrote:
> (Cc to dm-devel)
> 
> On 11/06/12 20:29, Jens Axboe wrote:
>> I recently fixed a deadlock for a customer we have with
>> the below patch. I have queued it up in my tree as not
>> to lose it. Can I have an ack from you, or do you want
>> to submit it yourself? I've marked it stable as well.
> 
> Could you tell details about how the deadlock happened?
> 
> dm's end_io always throws the completion handling to softirq:
>   end_clone_request()
>     dm_complete_request()
>       blk_complete_request()
> 
> then it is processed in softirq context:
>   dm_softirq_done()
>     dm_done()
>       dm_end_request()
>         rq_completed(run_queue=true)
>           blk_run_queue()
> 
> Since queue_lock is always held with interrupt disabled,
> I couldn't see why it could deadlock.
> 
> Request-based dm followed the completion model of scsi
> mid layer. So similar code path exists in scsi, too.
> For example:
>   scsi_request_fn()
>     scsi_kill_request()
>       blk_complete_request()
> then:
>   scsi_softirq_done()
>     scsi_io_completion()
>       scsi_next_command()
>         scsi_run_queue()
>           spin_lock queue_lock
>           __blk_run_queue()
> 
> If calling run-queue from softirq_done_fn() can cause deadlock,
> I'm afraid the problem is not limited to dm.

dm_softirq_done()
    rq_completed()
        blk_run_queue()
            dm_request_fn()
                dm_dispatch_request()
                    blk_insert_cloned_request()
                        __elv_add_request()
                            elv_insert()
                                blk_run_queue()
                                    ...

Basically you recurse back into the request handler, since it ends up
running the queue. But I see I've been too focused on the older release,
since we don't actually do that anymore after the plugging rewrite. So
it should actually be safe in current kernels and hence the patch only
needed for the stable series where that is not the case.

-- 
Jens Axboe