sg: fix EWOULDBLOCK errors with scsi-mq

With scsi-mq enabled, userspace programs can get unexpected EWOULDBLOCK
(a.k.a. EAGAIN) errors when submitting commands to the SCSI generic
driver.  Fix by calling blk_get_request() with GFP_KERNEL instead of
GFP_ATOMIC.

Note: to avoid introducing a potential deadlock, this patch should be
applied after the patch titled "sg: fix unkillable I/O wait deadlock
with scsi-mq".

Cc: <stable@vger.kernel.org> # 3.17+
Signed-off-by: Tony Battersby <tonyb@cybernetics.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Tested-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>

drivers/scsi/sg.c

@@ -1680,7 +1680,22 @@ sg_start_req(Sg_request *srp, unsigned char *cmd)
 			return -ENOMEM;
 	}
 
-	rq = blk_get_request(q, rw, GFP_ATOMIC);
+	/*
+	 * NOTE
+	 *
+	 * With scsi-mq enabled, there are a fixed number of preallocated
+	 * requests equal in number to shost->can_queue.  If all of the
+	 * preallocated requests are already in use, then using GFP_ATOMIC with
+	 * blk_get_request() will return -EWOULDBLOCK, whereas using GFP_KERNEL
+	 * will cause blk_get_request() to sleep until an active command
+	 * completes, freeing up a request.  Neither option is ideal, but
+	 * GFP_KERNEL is the better choice to prevent userspace from getting an
+	 * unexpected EWOULDBLOCK.
+	 *
+	 * With scsi-mq disabled, blk_get_request() with GFP_KERNEL usually
+	 * does not sleep except under memory pressure.
+	 */
+	rq = blk_get_request(q, rw, GFP_KERNEL);
 	if (IS_ERR(rq)) {
 		kfree(long_cmdp);
 		return PTR_ERR(rq);