[Bug 275570] self-referential nullfs mount over tmpfs in combination with MNT_UPDATE results in a hang

From: <bugzilla-noreply_at_freebsd.org>
Date: Wed, 13 Dec 2023 01:37:32 UTC
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=275570

Kyle Evans <kevans@freebsd.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dfr@FreeBSD.org,
                   |                            |kevans@freebsd.org

--- Comment #1 from Kyle Evans <kevans@freebsd.org> ---
Let's start with dfr@, since the sample in question is specifically
null-mounting a file over itself.

With the deadlock.c attached, I hit it best with a stupid simple loop:

```
#!/bin/sh

iteration=1

while true; do
        echo "Iteration $iteration"
        ./a.out
        iteration=$((iteration + 1))
done
```

Took me about 15 tries. When it deadlocks on my machine, it's in the nmount(2)
that would be null-mounting the file over itself rather than the follow-up
MNT_UPDATE of the tmpfs mount holding it.

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