[PATCH 3/3] xen-blkfront: If no barrier or flush is supported, use invalid operation.

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

By default we would set the info->flush_op=0, and zero maps
to BLKIF_OP_READ request code. So if the backend did not support
either barrier ('feature-barrier') or flush ('feature-flush-cache')
we would end up using that opcode for the flush/barrier request, meaning
we actually do a READ request. Fortunatly for us, __generic_make_request
checks q->flush_flags and realizes that we don't do FLUSH and removes
the REQ_FLUSH | REQ_FUA so we never end up issuing a READ request
for a flush request. However, other third party implementations of
__make_request might not be so smart, so lets fix this up.

CC: stable@kernel.org
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
 drivers/block/xen-blkfront.c |    8 ++++----
 1 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index e205d91..c4aa9da 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -97,7 +97,7 @@ struct blkfront_info
 	struct blk_shadow shadow[BLK_RING_SIZE];
 	unsigned long shadow_free;
 	unsigned int feature_flush;
-	unsigned int flush_op;
+	int flush_op;
 	unsigned int feature_discard;
 	unsigned int discard_granularity;
 	unsigned int discard_alignment;
@@ -774,7 +774,7 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
 				if (error == -EOPNOTSUPP)
 					error = 0;
 				info->feature_flush = 0;
-				info->flush_op = 0;
+				info->flush_op = -1; /* 0 is BLKIF_OP_READ */
 				xlvbd_flush(info);
 			}
 			/* fall through */
@@ -1207,7 +1207,7 @@ static void blkfront_connect(struct blkfront_info *info)
 	}
 
 	info->feature_flush = 0;
-	info->flush_op = 0;
+	info->flush_op = -1; /* 0 is BLKIF_OP_READ */
 
 	err = xenbus_gather(XBT_NIL, info->xbdev->otherend,
 			    "feature-barrier", "%d", &barrier,
@@ -1234,7 +1234,7 @@ static void blkfront_connect(struct blkfront_info *info)
 	if (!err && flush)
 		info->flush_op = BLKIF_OP_FLUSH_DISKCACHE;
 
-	if (info->flush_op)
+	if (info->flush_op > 0)
 		info->feature_flush = REQ_FLUSH | REQ_FUA;
 
 	err = xenbus_gather(XBT_NIL, info->xbdev->otherend,
-- 
1.7.4.1