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Date:      Wed, 13 Dec 2023 01:37:32 +0000
From:      bugzilla-noreply@freebsd.org
To:        fs@FreeBSD.org
Subject:   [Bug 275570] self-referential nullfs mount over tmpfs in combination with MNT_UPDATE results in a hang
Message-ID:  <bug-275570-3630-gCn9nmV9wC@https.bugs.freebsd.org/bugzilla/>
In-Reply-To: <bug-275570-3630@https.bugs.freebsd.org/bugzilla/>
References:  <bug-275570-3630@https.bugs.freebsd.org/bugzilla/>

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D275570

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=3D1

while true; do
        echo "Iteration $iteration"
        ./a.out
        iteration=3D$((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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