Messages in this thread Patch in this message | | | Date | Sat, 22 May 2010 16:52:20 +0200 | From | Tejun Heo <> | Subject | [PATCH] vfs: don't hold s_umount over close_bdev_exclusive() call |
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This patch fixes an obscure AB-BA deadlock in get_sb_bdev().
When a superblock is mounted more than once get_sb_bdev() calls close_bdev_exclusive() to drop the extra bdev reference while holding s_umount. However, sb->s_umount nests inside bd_mutex during __invalidate_device() and close_bdev_exclusive() acquires bd_mutex during blkdev_put(); thus creating an AB-BA deadlock.
This condition doesn't trigger frequently. For this condition to be visible to lockdep, the filesystem must occupy the whole device (as __invalidate_device() only grabs bd_mutex for the whole device), the FS must be mounted more than once and partition rescan should be issued while the FS is still mounted.
Fix it by dropping s_umount over close_bdev_exclusive().
Signed-off-by: Tejun Heo <tj@kernel.org> Reported-by: Ciprian Docan <docan@eden.rutgers.edu> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Al Viro <viro@zeniv.linux.org.uk> --- I think this fix is safe and seems to work fine here but I dunno know the locking too well, so it would be best not to push it w/o Al's ack.
Thanks.
fs/super.c | 9 +++++++++ 1 file changed, 9 insertions(+)
diff --git a/fs/super.c b/fs/super.c index 1527e6a..667f706 100644 --- a/fs/super.c +++ b/fs/super.c @@ -821,7 +821,16 @@ int get_sb_bdev(struct file_system_type *fs_type, goto error_bdev; }
+ /* + * s_umount nests inside bd_mutex during + * __invalidate_device(). close_bdev_exclusive() + * acquires bd_mutex and can't be called under + * s_umount. Drop s_umount temporarily. This is safe + * as we're holding an active reference. + */ + up_write(&s->s_umount); close_bdev_exclusive(bdev, mode); + down_write(&s->s_umount); } else { char b[BDEVNAME_SIZE];
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