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

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