From: "Ted Unangst" <tedu@tedunangst.com>
Subject: Re: Trying to understand vnode refcounting
To: "Martin Yoffe" <martin.yoffe@gmx.com>
Cc: tech@openbsd.org
Date: Thu, 15 May 2025 17:11:28 -0400

On 2025-05-15, Martin Yoffe wrote:

> At first I looked at VOP_LOOKUP(9) that describes how vnodes'
> refcounting should be handled before and after calling each operation.
> Then, out of curiosity, I tried to print some v_usecounts before
> and after calling VOP_* operations inside syscalls (on a ufs), and
> found some discrepancies with the information in VOP_LOOKUP(9). For
> example, the call to VOP_MKDIR in domkdirat (sys/kern/vfs_syscalls.c)
> decrements its dvp's v_usecount, which is not mentionned in the man
> page. Even weirder, when doing the same thing for the VOP_LOOKUP
> call inside vfs_lookup (sys/kern/vfs_lookup.c), sometimes dp's
> v_usecount is decremented, sometimes it stays the same.

When VOP_MKDIR says dvp is unlocked on exit, that means it called
vput which will drop the use count, too.