linux-kernel - [v2.6.34-stable 184/213] btrfs: use rcu_barrier() to wait for bdev puts at unmount

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Date:	Wed, 5 Feb 2014 15:02:19 -0500
From:	Paul Gortmaker <paul.gortmaker@...driver.com>
To:	<stable@...r.kernel.org>, <linux-kernel@...r.kernel.org>
CC:	Eric Sandeen <sandeen@...hat.com>,
	Josef Bacik <jbacik@...ionio.com>,
	Chris Mason <chris.mason@...ionio.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: [v2.6.34-stable 184/213] btrfs: use rcu_barrier() to wait for bdev puts at unmount

From: Eric Sandeen <sandeen@...hat.com>

                   -------------------
    This is a commit scheduled for the next v2.6.34 longterm release.
    http://git.kernel.org/?p=linux/kernel/git/paulg/longterm-queue-2.6.34.git
    If you see a problem with using this for longterm, please comment.
                   -------------------

commit bc178622d40d87e75abc131007342429c9b03351 upstream.

Doing this would reliably fail with -EBUSY for me:

# mount /dev/sdb2 /mnt/scratch; umount /mnt/scratch; mkfs.btrfs -f /dev/sdb2
...
unable to open /dev/sdb2: Device or resource busy

because mkfs.btrfs tries to open the device O_EXCL, and somebody still has it.

Using systemtap to track bdev gets & puts shows a kworker thread doing a
blkdev put after mkfs attempts a get; this is left over from the unmount
path:

btrfs_close_devices
	__btrfs_close_devices
		call_rcu(&device->rcu, free_device);
			free_device
				INIT_WORK(&device->rcu_work, __free_device);
				schedule_work(&device->rcu_work);

so unmount might complete before __free_device fires & does its blkdev_put.

Adding an rcu_barrier() to btrfs_close_devices() causes unmount to wait
until all blkdev_put()s are done, and the device is truly free once
unmount completes.

Signed-off-by: Eric Sandeen <sandeen@...hat.com>
Signed-off-by: Josef Bacik <jbacik@...ionio.com>
Signed-off-by: Chris Mason <chris.mason@...ionio.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@...driver.com>
---
 fs/btrfs/volumes.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index c04ebb1e4c98..7f75546941ac 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -565,6 +565,12 @@ int btrfs_close_devices(struct btrfs_fs_devices *fs_devices)
 		__btrfs_close_devices(fs_devices);
 		free_fs_devices(fs_devices);
 	}
+	/*
+	 * Wait for rcu kworkers under __btrfs_close_devices
+	 * to finish all blkdev_puts so device is really
+	 * free when umount is done.
+	 */
+	rcu_barrier();
 	return ret;
 }
 
-- 
1.8.5.2

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