Re: [PATCH V7 6/6] SCSI: set block queue at preempt only when SCSI device is put into quiesce

[Ming Lei]

On Mon, Oct 02, 2017 at 06:52:25AM -0700, Christoph Hellwig wrote:
> > +	/*
> > +	 * Simply quiesing SCSI device isn't safe, it is easy
> > +	 * to use up requests because all these allocated requests
> > +	 * can't be dispatched when device is put in QIUESCE.
> > +	 * Then no request can be allocated and we may hang
> > +	 * somewhere, such as system suspend/resume.
> > +	 *
> > +	 * So we set block queue in preempt only first, no new
> > +	 * normal request can enter queue any more, and all pending
> > +	 * requests are drained once blk_set_preempt_only()
> > +	 * returns. Only RQF_PREEMPT is allowed in preempt only mode.
> > +	 */
> > +	blk_set_preempt_only(sdev->request_queue, true);
> > +
> >  	mutex_lock(&sdev->state_mutex);
> >  	err = scsi_device_set_state(sdev, SDEV_QUIESCE);
> 
> Why is this not under state_mutex so that we guarantee it's atomic
> vs sdev state changes?

It isn't necessary since what we want is to make sure only RQF_PREEMPT
is allowed once blk_set_preempt_only() returns, and putting a lock
before freezing queue might risk to deadlock.

-- 
Ming