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Subject[229/264] blk-flush: fix invalid BUG_ON in blk_insert_flush
3.1-stable review patch.  If anyone has any objections, please let me know.

------------------
From: Jeff Moyer <jmoyer@redhat.com>

commit 834f9f61a525d2f6d3d0c93894e26326c8d3ceed upstream.

A user reported a regression due to commit
4853abaae7e4a2af938115ce9071ef8684fb7af4 (block: fix flush
machinery for stacking drivers with differring flush flags).
Part of the problem is that blk_insert_flush required a
single bio be attached to the request. In reality, having
no attached bio is also a valid case, as can be observed with
an empty flush.

[1] http://www.redhat.com/archives/dm-devel/2011-September/msg00154.html

Reported-by: Christophe Saout <christophe@saout.de>
Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
block/blk-flush.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
--- a/block/blk-flush.c
+++ b/block/blk-flush.c
@@ -320,7 +320,7 @@ void blk_insert_flush(struct request *rq
return;
}

- BUG_ON(!rq->bio || rq->bio != rq->biotail);
+ BUG_ON(rq->bio != rq->biotail); /*assumes zero or single bio rq */

/*
* If there's data but flush is not necessary, the request can be



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