[PATCH] Avoid dereferencing a 'request_queue' after last close.

[NeilBrown]

On the last close of an 'md' device which as been stopped, the device
is destroyed and in particular the request_queue is freed.  The free
is done in a separate thread so it might happen a short time later.

__blkdev_put calls bdev_inode_switch_bdi *after* ->release has been
called.

Since commit f758eeabeb96f878c860e8f110f94ec8820822a9
bdev_inode_switch_bdi will dereference the 'old' bdi, which lives
inside a request_queue, to get a spin lock.  This causes the last
close on an md device to sometime take a spin_lock which lives in
freed memory - which results in an oops.

So move the called to bdev_inode_switch_bdi before the call to
->release.

Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Wu Fengguang <fengguang.wu@xxxxxxxxx>
Cc: stable@xxxxxxxxxx
Signed-off-by: NeilBrown <neilb@xxxxxxx>

diff --git a/fs/block_dev.c b/fs/block_dev.c
index ff77262..d8a753f 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -1430,6 +1430,12 @@ static int __blkdev_put(struct block_device *bdev, fmode_t mode, int for_part)
 		sync_blockdev(bdev);
 		kill_bdev(bdev);
 	}
+	if (!bdev->bd_openers)
+		/* ->release can cause the old bdi to disappear,
+		 * so must switch it out first
+		 */
+		bdev_inode_switch_bdi(bdev->bd_inode,
+					&default_backing_dev_info);
 	if (bdev->bd_contains == bdev) {
 		if (disk->fops->release)
 			ret = disk->fops->release(disk, mode);
@@ -1442,8 +1448,6 @@ static int __blkdev_put(struct block_device *bdev, fmode_t mode, int for_part)
 		disk_put_part(bdev->bd_part);
 		bdev->bd_part = NULL;
 		bdev->bd_disk = NULL;
-		bdev_inode_switch_bdi(bdev->bd_inode,
-					&default_backing_dev_info);
 		if (bdev != bdev->bd_contains)
 			victim = bdev->bd_contains;
 		bdev->bd_contains = NULL;
--
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