2d4422e799
UFS uses a new "mntfs" pseudo file system which provides private device vnodes for a file system to safely access its disk device. The original device vnode is saved in um_odevvp to hold the exclusive lock on the device so that any attempts to open it for writing will fail. But it is otherwise unused and has its BO_NOBUFS flag set to enforce that file systems using mntfs vnodes do not accidentally use the original devfs vnode. When the file system is unmounted, um_odevvp is no longer needed and is released. The lock order reversal happens because device vnodes must be locked before UFS vnodes. During unmount, the root directory vnode lock is held. When when calling vrele() on um_odevvp, vrele() attempts to exclusive lock um_odevvp causing the lock order reversal. The problem is eliminated by doing a non-blocking exclusive lock on um_odevvp which will always succeed since there are no users of um_odevvp. With um_odevvp locked, it can be released using vput which does not attempt to do a blocking exclusive lock request and thus avoids the lock order reversal. Sponsored by: Netflix |
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