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with snapshots. Each vnode has an embedded lock that controls access to its contents. However vnodes describing a UFS snapshot all share a single snapshot lock to coordinate their access and update. As part of mounting a UFS filesystem with snapshots, each of the vnodes describing a snapshot has its individual lock replaced with the snapshot lock. When the filesystem is unmounted the vnode's original lock is returned replacing the snapshot lock. The lock order reversal happens because vnode locks must be acquired before snapshot locks. When unmounting we must lock both the snapshot lock and the vnode lock before swapping them so that the vnode will be continuously locked during the swap. For each vnode representing a snapshot, we must first acquire the snapshot lock to ensure exclusive access to it and its original lock. We then face a lock order reversal when we try to acquire the original vnode lock. The problem is eliminated by doing a non-blocking exclusive lock on the original lock which will always succeed since there are no users of that lock. Sponsored by: Netflix |
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