Re: [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:

quoted

+	/*
+	 * 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