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SubjectRe: [PATCH] vfs: don't hold s_umount over close_bdev_exclusive() call
On Sat, May 22 2010, Tejun Heo wrote:
> 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().

Looks safe to me, since it has (as you note) an elevated ref count.

Acked-by: Jens Axboe <jens.axboe@oracle.com>


--
Jens Axboe



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