[PATCH] block: fix DISCARD_BARRIER requests

[Christoph Hellwig <hch@lst.de>]

Filesystems assume that DISCARD_BARRIER are full barriers, so that they
don't have to track in-progress discard operation when submitting new I/O.
But currently we only treat them as elevator barriers, which don't
actually do the nessecary queue drains.

Also remove the unlikely around both the DISCARD and BARRIER requests -
the happen far too often for a static mispredict.

Signed-off-by: Christoph Hellwig <hch@lst.de>

Index: linux-2.6/block/blk-core.c
===================================================================
--- linux-2.6.orig/block/blk-core.c	2010-06-17 09:42:23.924004448 +0200
+++ linux-2.6/block/blk-core.c	2010-06-17 09:45:46.727004030 +0200
@@ -1149,13 +1149,10 @@ void init_request_from_bio(struct reques
 	else
 		req->cmd_flags |= bio->bi_rw & REQ_FAILFAST_MASK;
 
-	if (unlikely(bio_rw_flagged(bio, BIO_RW_DISCARD))) {
+	if (bio_rw_flagged(bio, BIO_RW_DISCARD))
 		req->cmd_flags |= REQ_DISCARD;
-		if (bio_rw_flagged(bio, BIO_RW_BARRIER))
-			req->cmd_flags |= REQ_SOFTBARRIER;
-	} else if (unlikely(bio_rw_flagged(bio, BIO_RW_BARRIER)))
+	if (bio_rw_flagged(bio, BIO_RW_BARRIER))
 		req->cmd_flags |= REQ_HARDBARRIER;
-
 	if (bio_rw_flagged(bio, BIO_RW_SYNCIO))
 		req->cmd_flags |= REQ_RW_SYNC;
 	if (bio_rw_flagged(bio, BIO_RW_META))