Re: [PATCH v4] blk-mq: Fix race conditions in request timeout handling

[Ming Lei <ming.lei@redhat.com>]

Hi Tejun,

On Tue, Apr 10, 2018 at 08:30:31AM -0700, tj@kernel.org wrote:
> Hello, Ming.
> 
> On Tue, Apr 10, 2018 at 11:25:54PM +0800, Ming Lei wrote:
> > +   if (time_after_eq(jiffies, deadline) &&
> > +       blk_mq_change_rq_state(rq, MQ_RQ_IN_FLIGHT, MQ_RQ_COMPLETE)) {
> > +       blk_mq_rq_timed_out(rq, reserved);
> > 
> > Normal completion still can happen between blk_mq_change_rq_state()
> > and blk_mq_rq_timed_out().
> > 
> > In tj's approach, there is synchronize_rcu() between writing aborted_gstate
> > and blk_mq_rq_timed_out, it is easier for normal completion to happen during
> > the big window.
> 
> I don't think plugging this hole is all that difficult, but this
> shouldn't lead to any critical failures.  If so, that'd be a driver
> bug.

I agree, the issue should be in driver's irq handler and .timeout in
theory.

For example, even though one request has been done by irq handler, .timeout
still may return RESET_TIMER.

Thanks,
Ming