linux-kernel - Re: [PATCH] fs/9p: fix inode nlink accounting

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Date: Mon, 08 Jan 2024 13:08:31 +0100
From: Christian Schoenebeck <linux_oss@...debyte.com>
To: Eric Van Hensbergen <ericvh@...nel.org>, asmadeus@...ewreck.org
Cc: linux-kernel@...r.kernel.org, v9fs@...ts.linux.dev, rminnich@...il.com,
 lucho@...kov.net
Subject: Re: [PATCH] fs/9p: fix inode nlink accounting

On Monday, January 8, 2024 12:19:34 PM CET asmadeus@...ewreck.org wrote:
> Eric Van Hensbergen wrote on Sun, Jan 07, 2024 at 07:07:52PM +0000:
> > I was running some regressions and noticed a (race-y) kernel warning that
> > happens when nlink becomes less than zero.  Looking through the code
> > it looks like we aren't good about protecting the inode lock when
> > manipulating nlink and some code that was added several years ago to
> > protect against bugs in underlying file systems nlink handling didn't
> > look quite right either.  I took a look at what NFS was doing and tried to
> > follow similar approaches in the 9p code.
> 
> I was about to say the set/inc/etc_nlink helpers could probably just be
> using atomic (there's an atomic_dec_if_postive that we could have used
> for the v9fs_dec_count warning), but this isn't our code so not much to
> do about that -- I agree it needs a lock.
> 
> I didn't take the time to check if you missed any, but it won't be worse
> than what we have right now:
> Acked-by: Dominique Martinet <asmadeus@...ewreck.org>

That's actually a good point. For these tasks atomic inc/sub/etc are usually
used instead of locks.

I would at least add local wrapper functions that would do these spinlocks for
us.

However would it be too bold to change those inode functions to use atomic
operations directly on their end?

/Christian