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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>
This commit is contained in:
Tony Battersby 2015-02-13 12:10:58 -05:00 committed by James Bottomley
parent 7568615c10
commit 7772855a99
1 changed files with 16 additions and 1 deletions
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17
drivers/scsi/sg.c
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@ -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);