[PATCH] block: fix request sorting at unplug

[PATCH] block: fix request sorting at unplug

[Konstantin Khlebnikov]

Comparison function for list_sort() must be anticommutative,
otherwise it is not sorting in ordinary meaning.

But fortunately list_sort() always check ((*cmp)(priv, a, b) <= 0)
it not distinguish negative and zero, so comparison function can
implement only less-or-equal instead of full three-way comparison.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
---
 block/blk-core.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index e0a0623..5e151d2 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -2665,7 +2665,7 @@ static int plug_rq_cmp(void *priv, struct list_head *a, struct list_head *b)
 	struct request *rqa = container_of(a, struct request, queuelist);
 	struct request *rqb = container_of(b, struct request, queuelist);
 
-	return !(rqa->q == rqb->q);
+	return !(rqa->q <= rqb->q);
 }
 
 static void flush_plug_list(struct blk_plug *plug)

Re: [PATCH] block: fix request sorting at unplug

[Jens Axboe]

On 2011-04-02 17:37, Konstantin Khlebnikov wrote:
> Comparison function for list_sort() must be anticommutative,
> otherwise it is not sorting in ordinary meaning.
> 
> But fortunately list_sort() always check ((*cmp)(priv, a, b) <= 0)
> it not distinguish negative and zero, so comparison function can
> implement only less-or-equal instead of full three-way comparison.

Thanks, good catch! It's an optimization, so no breakage. But if you end
up often having > 1 queue plugged, it's a good optimization.

-- 
Jens Axboe