Re: [PATCH] blk-mq: avoid to iterate over stale request

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

On Mon, Sep 06, 2021 at 03:44:08PM -0700, Bart Van Assche wrote:
> On 9/5/21 23:50, Ming Lei wrote:
> > -	if (!rq || !refcount_inc_not_zero(&rq->ref))
> > +	if (!rq || rq->tag != bitnr || !refcount_inc_not_zero(&rq->ref))
> >   		rq = NULL;
> 
> Shouldn't the rq->tag != bitnr test happen after the refcount has been
> incremented since otherwise rq->tag can change after it has been read and
> before the refcount is incremented?

rq->tag can change too after its refcount is grabbed. If the rq is released
during the iterating, either SCMD_STATE_INFLIGHT is cleared or
refcount_inc_not_zero() fails. So this way works.

The use case for scsi_host_queue_ready() and scsi EH handling is a bit
special. For others, either the iterating needn't to be exact, or queue
is frozen.



Thanks,
Ming