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>
diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
index 208bf3c..5cb2a5e 100644
--- a/drivers/scsi/sg.c
+++ b/drivers/scsi/sg.c
@@ -1680,7 +1680,22 @@
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);