From: Eric Sandeen <sandeen@redhat.com>
Subject: [RHEL5.1 PATCH] BZ 225435, elevator: move clearing of unplug flag  earlier
Date: Thu, 29 Mar 2007 16:21:51 -0500
Bugzilla: 225435
Message-Id: <460C2DEF.4070702@redhat.com>
Changelog: [elevator] move clearing of unplug flag  earlier


For Bugzilla Bug 225435: LTC31512-Backport of patch to elevator code

Changelog below says it well.  It's in Linus' tree since late January.

Thanks,

-Eric

------------

X-Git-Tag: v2.6.20-rc6~28
X-Git-Url: http://git.kernel.org/?p=linux%2Fkernel%2Fgit%2Ftorvalds%2Flinux-2.6.git;a=commitdiff_plain;h=95543179f158b4891c5dc49004853ce081e8d794

[PATCH] elevator: move clearing of unplug flag earlier

A flag was recently added to the elevator code to avoid
performing an unplug when reuests are being re-queued.
The goal of this flag was to avoid a deep recursion that
can occur when re-queueing requests after a SCSI device/host
reset.  See http://lkml.org/lkml/2006/5/17/254

However, that fix added the flag near the bottom of a case
statement, where an earlier break (in an if statement) could
transport one out of the case, without setting the flag.
This patch sets the flag earlier in the case statement.

I re-discovered the deep recursion recently during testing;
I was told that it was a known problem, and the fix to it was
in the kernel I was testing. Indeed it was ... but it didn't
fix the bug. With the patch below, I no longer see the bug.

Signed-off by: Linas Vepstas <linas@austin.ibm.com>
Signed-off-by: Jens Axboe <axboe@suse.de>
Cc: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---

Index: linux-2.6.18-13.el5/block/elevator.c
===================================================================
--- linux-2.6.18-13.el5.orig/block/elevator.c
+++ linux-2.6.18-13.el5/block/elevator.c
@@ -389,6 +389,12 @@ void elv_insert(request_queue_t *q, stru
 		 */
 		rq->flags |= REQ_SOFTBARRIER;
 
+		/*
+		 * Most requeues happen because of a busy condition,
+		 * don't force unplug of the queue for that case.
+		 */
+		unplug_it = 0;
+
 		if (q->ordseq == 0) {
 			list_add(&rq->queuelist, &q->queue_head);
 			break;
@@ -403,11 +409,6 @@ void elv_insert(request_queue_t *q, stru
 		}
 
 		list_add_tail(&rq->queuelist, pos);
-		/*
-		 * most requeues happen because of a busy condition, don't
-		 * force unplug of the queue for that case.
-		 */
-		unplug_it = 0;
 		break;
 
 	default: