Re: [PATCH] block: mq flush: fix race between IPI handler and mq flush worker

[Christoph Hellwig <hch@lst.de>]

On Wed, May 21, 2014 at 01:16:14PM +0800, Ming Lei wrote:
> I am wondering if virtio-blk is trivial block driver, :-)

It's about as simple as it gets.

> > The scsi-mq work that I plant to submit for the next merge window is
> > the prime example.
> 
> It depends if one scsi-mq req has to requeue itself with rq->requeue_work
> inside its own .softirq_done_fn. If yes, we can't put call_single_data
> and requeue_work into one union simply. From you last scsi-mq post,
> looks the request may do that if I understand correctly.

Requeueing a request from the completion handler is indeed what we'll
need with various more complete drivers.

> I think the patch is clean and simple, with documenting the special
> conflict case clearly too.

While I can't say anything against the fact that it fixes the issue
it's neither clean nor simple.

> Follows current ideas:
> 1), this patch with scsi-mq sharing abort_work together?
> 2), move requeue_work out of the union inside request
> 3), spin_lock_irqsave(&ctx->lock) everywhere and requeue
> request directly to ctx without using work

I think Jens very much wanted to avoid irq disabling in the I/O path
if possible. If we have a separate requeue list with it's separate
lock we can avoid that unless we actually have to take requests of
that requeue list.  I can look into that implementation.