[dm-devel] [PATCH] scsi: Fix dm-multipath starvation when scsi host is busy

[Jun'ichi Nomura]

When a scsi host is kept busy by a dm-mpath device, other dm-mpath device
on the same host could be starved. For example:
http://www.redhat.com/archives/dm-devel/2012-May/msg00123.html

It happens because dm-mpath delays request submission when the underlying
scsi host is busy even if sdev is not busy.

For case like this, it is better to send the request down and let scsi do
appropriate starvation control over the shared resource.

Though it might seem odd to change scsi's definition of a LLD being "busy",
it is reasonable because scsi_lld_busy (and blk_lld_busy) was introduced
to provide a hint for request-based stacking driver (i.e. dm-multipath)
and dm-multipath is the only user of this function.

Reported-by: Bernd Schubert <bernd.schubert at itwm.fraunhofer.de>
Tested-by: Bernd Schubert <bernd.schubert at itwm.fraunhofer.de>
Signed-off-by: Jun'ichi Nomura <j-nomura at ce.jp.nec.com>
Cc: James Bottomley <James.Bottomley at HansenPartnership.com>
Cc: Mike Snitzer <snitzer at redhat.com>
Cc: Alasdair G Kergon <agk at redhat.com>
Cc: <stable at vger.kernel.org>
---
 drivers/scsi/scsi_lib.c |    5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 5dfd749..0eb4602 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -1378,16 +1378,13 @@ static int scsi_lld_busy(struct request_queue *q)
 {
 	struct scsi_device *sdev = q->queuedata;
 	struct Scsi_Host *shost;
-	struct scsi_target *starget;
 
 	if (!sdev)
 		return 0;
 
 	shost = sdev->host;
-	starget = scsi_target(sdev);
 
-	if (scsi_host_in_recovery(shost) || scsi_host_is_busy(shost) ||
-	    scsi_target_is_busy(starget) || scsi_device_is_busy(sdev))
+	if (scsi_host_in_recovery(shost) || scsi_device_is_busy(sdev)) 
 		return 1;
 
 	return 0;