Re: blk-mq crash under KVM in multiqueue block code (with virtio-blk and ext4)

[David Hildenbrand <dahi@linux.vnet.ibm.com>]

> On Wed, Sep 17, 2014 at 10:22 PM, Jens Axboe <axboe@kernel.dk> wrote:
> >
> > Another way would be to ensure that the timeout handler doesn't touch hw_ctx
> > or tag_sets that aren't fully initialized yet. But I think this is
> > safer/cleaner.
> 
> That may not be easy or enough to check if hw_ctx/tag_sets are
> fully initialized if you mean all requests have been used one time.
> 
> On Wed, Sep 17, 2014 at 10:11 PM, David Hildenbrand
> > I was playing with a simple patch that just sets cmd_flags and action_flags to
> 
> What is action_flags?

atomic_flags, sorry :)

Otherwise e.g. REQ_ATOM_STARTED could already be set due to the randomness. I
am not sure if this is really necessary, or if it is completely shielded by the
tag-handling code, but seemed to be clean for me to do it (and I remember it
not being set within blk_mq_rq_ctx_init).

> 
> > 0. That should already be sufficient to hinder blk_mq_tag_to_rq and the calling
> > method to do the wrong thing.
> 
> Yes, clearing rq->cmd_flags should be enough.
> 
> And looks better to move rq initialization to __blk_mq_free_request()
> too, otherwise timeout still may see old cmd_flags and rq->q before
> rq's new initialization.

Yes, __blk_mq_free_request() should also reset at least rq->cmd_flags, and I
think we can remove the initialization from  __blk_mq_alloc_request().

David

> 
> 
> Thanks,