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ufs: handle LoR between snap lock and vnode lock
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Authored by mckusick on Jan 19 2022, 6:58 AM.

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Summary

When a filesystem is mounted all of its associated snapshots must be activated. It first allocates a snapshot lock (snaplk) that will be shared by all the snapshot vnodes associated with the filesystem. As part of each snapshot file activation, it must replace its own ufs vnode lock with the snaplk. In this way acquiring the snaplk gives exclusive access to all the snapshots for the filesystem.

A write to a ufs vnode first acquires the ufs vnode lock for the file to be written then acquires the snaplk. Once it has the snaplk, it can check all the snapshots to see if any of them needs to make a copy of the block that is about to be written. This ffs_copyonwrite() code path establishes the ufs vnode followed by snaplk locking order.

When a filesystem is unmounted it has to release all of its snapshot vnodes. Part of doing the release is to revert the snapshot vnode from using the snaplk to using its original vnode lock. While holding the snaplk, the vnode lock has to be acquired, the vnode updated to reference it, then the snaplk released. Acquiring the vnode lock while holding the snaplk violates the ufs vnode then snaplk order. Because the vnode lock is unused, using LK_EXCLUSIVE | LK_NOWAIT to acquire it will always succeed and the LK_NOWAIT prevents the reverse lock order from being recorded.

This change was made in January 2021 (173779b98f) to avoid an LOR violation in ffs_snapshot_unmount(). The same LOR issue was recently found again when removing a snapshot in ffs_snapremove() which must also revert the snaplk to the original vnode lock as part of freeing it.

The unwind in ffs_snapremove() deals with the case in which the snaplk is held as a recursive lock holding multiple references. Specifically an equal number of references are made on the vnode lock. This change factors out the lock reversion operations into a new function revert_snaplock() which handles both the recursive locks and avoids the LOR. The new revert_snaplock() function is then used in both ffs_snapshot_unmount() and in ffs_snapremove().

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