backing-dev: ensure that a removed bdi no longer has super_block referencing it
When the bdi is being removed, we have to ensure that no super_blocks currently have that cached in sb->s_bdi. Normally this is ensured by the sb having a longer life span than the bdi, but if the device is suddenly yanked, we have to kill this reference. sb->s_bdi is pointed to freed memory at that point. This fixes a problem with sync(1) hanging when a USB stick is pulled without cleanly umounting it first. Reported-by: Pavel Machek <pavel@ucw.cz> Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
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