This is a note to let you know that I've just added the patch titled sg: fix EWOULDBLOCK errors with scsi-mq to the 3.18-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: sg-fix-ewouldblock-errors-with-scsi-mq.patch and it can be found in the queue-3.18 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 7772855a996ec6e16944b120ab5ce21050279821 Mon Sep 17 00:00:00 2001 From: Tony Battersby <tonyb@xxxxxxxxxxxxxxx> Date: Fri, 13 Feb 2015 12:10:58 -0500 Subject: sg: fix EWOULDBLOCK errors with scsi-mq From: Tony Battersby <tonyb@xxxxxxxxxxxxxxx> commit 7772855a996ec6e16944b120ab5ce21050279821 upstream. 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". Signed-off-by: Tony Battersby <tonyb@xxxxxxxxxxxxxxx> Acked-by: Douglas Gilbert <dgilbert@xxxxxxxxxxxx> Tested-by: Douglas Gilbert <dgilbert@xxxxxxxxxxxx> Signed-off-by: James Bottomley <JBottomley@xxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- drivers/scsi/sg.c | 17 ++++++++++++++++- 1 file changed, 16 insertions(+), 1 deletion(-) --- a/drivers/scsi/sg.c +++ b/drivers/scsi/sg.c @@ -1721,7 +1721,22 @@ sg_start_req(Sg_request *srp, unsigned c 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); Patches currently in stable-queue which might be from tonyb@xxxxxxxxxxxxxxx are queue-3.18/blk-mq-fix-double-free-in-error-path.patch queue-3.18/sg-fix-ewouldblock-errors-with-scsi-mq.patch queue-3.18/sg-fix-unkillable-i-o-wait-deadlock-with-scsi-mq.patch -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html