[PATCH] xen-blkfront: fix kernel panic with negotiate_mq error path

[Manjunath Patil <manjunath.b.patil@oracle.com>]

info->nr_rings isn't adjusted in case of ENOMEM error from
negotiate_mq(). This leads to kernel panic in error path.

Typical call stack involving panic -
 #8 page_fault at ffffffff8175936f
    [exception RIP: blkif_free_ring+33]
    RIP: ffffffffa0149491  RSP: ffff8804f7673c08  RFLAGS: 00010292
 ...
 #9 blkif_free at ffffffffa0149aaa [xen_blkfront]
 #10 talk_to_blkback at ffffffffa014c8cd [xen_blkfront]
 #11 blkback_changed at ffffffffa014ea8b [xen_blkfront]
 #12 xenbus_otherend_changed at ffffffff81424670
 #13 backend_changed at ffffffff81426dc3
 #14 xenwatch_thread at ffffffff81422f29
 #15 kthread at ffffffff810abe6a
 #16 ret_from_fork at ffffffff81754078

Though either of my changes avoid the panic, I included both the
changes. This issue got introduced with "7ed8ce1 xen-blkfront: move
negotiate_mq to cover all cases of new VBDs"

Signed-off-by: Manjunath Patil <manjunath.b.patil@oracle.com>
---
 drivers/block/xen-blkfront.c |    7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 429d201..dc8fe25 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -1350,8 +1350,10 @@ static void blkif_free(struct blkfront_info *info, int suspend)
 	if (info->rq)
 		blk_mq_stop_hw_queues(info->rq);
 
-	for (i = 0; i < info->nr_rings; i++)
-		blkif_free_ring(&info->rinfo[i]);
+	if (info->rinfo) {
+		for (i = 0; i < info->nr_rings; i++)
+			blkif_free_ring(&info->rinfo[i]);
+	}
 
 	kfree(info->rinfo);
 	info->rinfo = NULL;
@@ -1919,6 +1921,7 @@ static int negotiate_mq(struct blkfront_info *info)
 			      GFP_KERNEL);
 	if (!info->rinfo) {
 		xenbus_dev_fatal(info->xbdev, -ENOMEM, "allocating ring_info structure");
+		info->nr_rings = 0;
 		return -ENOMEM;
 	}
 
-- 
1.7.1