Patch "scsi: virtio_scsi: Reject commands when virtqueue is broken" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

scsi: virtio_scsi: Reject commands when virtqueue is broken

to the 4.4-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:
scsi-virtio_scsi-reject-commands-when-virtqueue-is-broken.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Eric Farman <[email protected]>
Date: Fri, 13 Jan 2017 12:48:06 -0500
Subject: scsi: virtio_scsi: Reject commands when virtqueue is broken

From: Eric Farman <[email protected]>

[ Upstream commit 773c7220e22d193e5667c352fcbf8d47eefc817f ]

In the case of a graceful set of detaches, where the virtio-scsi-ccw disk is removed from the guest prior to the controller, the guest behaves quite normally. Specifically, the detach gets us into sd_sync_cache to issue a Synchronize Cache(10) command, which immediately fails (and is retried a couple of times) because the device has been removed. Later, the removal of the controller sees two CRWs presented, but there's no further indication of the removal from the guest viewpoint.

 [   17.217458] sd 0:0:0:0: [sda] Synchronizing SCSI cache
 [   17.219257] sd 0:0:0:0: [sda] Synchronize Cache(10) failed: Result: hostbyte=DID_BAD_TARGET driverbyte=DRIVER_OK
 [   21.449400] crw_info : CRW reports slct=0, oflw=0, chn=1, rsc=3, anc=0, erc=4, rsid=2
 [   21.449406] crw_info : CRW reports slct=0, oflw=0, chn=0, rsc=3, anc=0, erc=4, rsid=0

However, on s390, the SCSI disks can be removed "by surprise" when an entire controller (host) is removed and all associated disks are removed via the loop in scsi_forget_host. The same call to sd_sync_cache is made, but because the controller has already been removed, the Synchronize Cache(10) command is neither issued (and then failed) nor rejected.

That the I/O isn't returned means the guest cannot have other devices added nor removed, and other tasks (such as shutdown or reboot) issued by the guest will not complete either. The virtio ring has already been marked as broken (via virtio_break_device in virtio_ccw_remove), but we still attempt to queue the command only to have it remain there. The calling sequence provides a bit of distinction for us:

virtscsi_queuecommand()
-> virtscsi_kick_cmd()

    -> virtscsi_add_cmd()
     -> virtqueue_add_sgs()
      -> virtqueue_add()
         if success
           return 0
         elseif vq->broken or vring_mapping_error()
           return -EIO
         else
           return -ENOSPC

A return of ENOSPC is generally a temporary condition, so returning "host busy" from virtscsi_queuecommand makes sense here, to have it redriven in a moment or two. But the EIO return code is more of a permanent error and so it would be wise to return the I/O itself and allow the calling thread to finish gracefully. The result is these four kernel messages in the guest (the fourth one does not occur prior to this patch):

 [   22.921562] crw_info : CRW reports slct=0, oflw=0, chn=1, rsc=3, anc=0, erc=4, rsid=2
 [   22.921580] crw_info : CRW reports slct=0, oflw=0, chn=0, rsc=3, anc=0, erc=4, rsid=0
 [   22.921978] sd 0:0:0:0: [sda] Synchronizing SCSI cache
 [   22.921993] sd 0:0:0:0: [sda] Synchronize Cache(10) failed: Result: hostbyte=DID_BAD_TARGET driverbyte=DRIVER_OK

I opted to fill in the same response data that is returned from the more graceful device detach, where the disk device is removed prior to the controller device.

---
 drivers/scsi/virtio_scsi.c |   11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

@@ -533,7 +533,9 @@ static int virtscsi_queuecommand(struct
 {
 	struct Scsi_Host *shost = virtio_scsi_host(vscsi->vdev);
 	struct virtio_scsi_cmd *cmd = scsi_cmd_priv(sc);
+	unsigned long flags;
 	int req_size;
+	int ret;
 
 	BUG_ON(scsi_sg_count(sc) > shost->sg_tablesize);
 

@@ -561,8 +563,15 @@ static int virtscsi_queuecommand(struct
 		req_size = sizeof(cmd->req.cmd);
 	}
 
-	if (virtscsi_kick_cmd(req_vq, cmd, req_size, sizeof(cmd->resp.cmd)) != 0)
+	ret = virtscsi_kick_cmd(req_vq, cmd, req_size, sizeof(cmd->resp.cmd));
+	if (ret == -EIO) {
+		cmd->resp.cmd.response = VIRTIO_SCSI_S_BAD_TARGET;
+		spin_lock_irqsave(&req_vq->vq_lock, flags);
+		virtscsi_complete_cmd(vscsi, cmd);
+		spin_unlock_irqrestore(&req_vq->vq_lock, flags);
+	} else if (ret != 0) {
 		return SCSI_MLQUEUE_HOST_BUSY;
+	}
 	return 0;
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/scsi-virtio_scsi-reject-commands-when-virtqueue-is-broken.patch