Re: [PATCH v2 2/2] [SCSI] sg: fix EWOULDBLOCK errors with scsi-mq

From: Douglas Gilbert
Date: Sun Feb 15 2015 - 17:12:17 EST

On 15-02-13 12:10 PM, Tony Battersby wrote:
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

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: Douglas Gilbert <dgilbert@xxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # 3.17+
Signed-off-by: Tony Battersby <tonyb@xxxxxxxxxxxxxxx>

Acked-by: Douglas Gilbert <dgilbert@xxxxxxxxxxxx>
Tested-by: Douglas Gilbert <dgilbert@xxxxxxxxxxxx>

For inclusion in kernel 3.20.

The difference in behavior is due to bt_get() in block/blk-mq-tag.c
checking for __GFP_WAIT.

The bsg driver already calls blk_get_request() with GFP_KERNEL, so there
is no need for a change there.

--- linux-3.19.0/drivers/scsi/sg.c.orig 2015-02-13 11:04:40.000000000 -0500
+++ linux-3.19.0/drivers/scsi/sg.c 2015-02-13 11:05:14.000000000 -0500
@@ -1695,7 +1695,22 @@ sg_start_req(Sg_request *srp, unsigned c
return -ENOMEM;

+ /*
+ * 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);
- rq = blk_get_request(q, rw, GFP_ATOMIC);
if (IS_ERR(rq)) {
return PTR_ERR(rq);


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