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>

1 files changed, 16 insertions, 1 deletions

diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
index 208bf3c8a16c..5cb2a5e80b2b 100644
--- a/drivers/scsi/sg.c
+++ b/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);