Re: [PATCH] Fix deadlock with request based dm and some drivers

On 2012-11-07 01:15, Jun'ichi Nomura wrote:

On 11/06/12 21:35, Jens Axboe wrote:

quoted

On 2012-11-06 13:14, Jun'ichi Nomura wrote:

quoted

(Cc to dm-devel)

On 11/06/12 20:29, Jens Axboe wrote:

quoted

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()

          ^^ this is for dm's queue

quoted

            dm_request_fn()
                dm_dispatch_request()
                    blk_insert_cloned_request()
                        __elv_add_request()
                            elv_insert()
                                blk_run_queue()

                                  ^^ this is for lower device's queue

quoted

                                    ...

Basically you recurse back into the request handler, since it ends up
running the queue.

But the queues are different as commented inline above.
So it should be ok. (from deadlock point of view)

It's still not OK, some drivers end up doing spin_unlock_irq() in their
request_fn. Running unknown request_fn from ipi/irq is a bad idea, imho,
it'll quickly cause problems.

quoted

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.

BTW, using blk_run_queue_async() in softirq_done_fn() might be good
from performance point of view? It may reduce latency of the softirq
and have an effect of batching request_fn calls.

Yes, I argued the same for the original people who saw the problem. It
is quite an extensive and expensive path to have off the IPI handler. So
from that point of view, I'd recommend the patch as well.

-- 
Jens Axboe