linux-kernel - Re: [PATCH RFC 3/4] lockref: rework CMPXCHG

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Message-ID: <d011c2c46732cc0794e787196d71fb90477ff4b8.camel@kernel.org>
Date: Mon, 05 Aug 2024 08:52:28 -0400
From: Jeff Layton <jlayton@...nel.org>
To: Christian Brauner <brauner@...nel.org>
Cc: Mateusz Guzik <mjguzik@...il.com>, Alexander Viro
 <viro@...iv.linux.org.uk>,  Jan Kara <jack@...e.cz>, Andrew Morton
 <akpm@...ux-foundation.org>, Josef Bacik <josef@...icpanda.com>, 
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 3/4] lockref: rework CMPXCHG_LOOP to handle
 contention better

On Mon, 2024-08-05 at 13:44 +0200, Christian Brauner wrote:
> > Audit not my favorite area of the kernel to work in either. I don't see
> > a good way to make it rcu-friendly, but I haven't looked too hard yet
> > either. It would be nice to be able to do some of the auditing under
> > rcu or spinlock.
> 
> For audit your main option is to dodge the problem and check whether
> audit is active and only drop out of rcu if it is. That sidesteps the
> problem. I'm somewhat certain that a lot of systems don't really have
> audit active.
> 

I did have an earlier version of 4/4 that checked audit_context() and
stayed in RCU mode if it comes back NULL. I can resurrect that if you
think it's worthwhile.

> From a brief look at audit it would be quite involved to make it work
> just under rcu. Not just because it does various allocation but it also
> reads fscaps from disk and so on. That's not going to work unless we add
> a vfs based fscaps cache similar to what we do for acls. I find that
> very unlikely. 

Yeah. It wants to record a lot of (variable-length) information at very
inconvenient times. I think we're sort of stuck with it though until
someone has a vision on how to do this in a non-blocking way.

Handwavy thought: there is some similarity to tracepoints in what
audit_inode does, and tracepoints are able to be called in all sorts of
contexts. I wonder if we could leverage the same infrastructure
somehow? The catch here is that we can't just drop audit records if
things go wrong.

-- 
Jeff Layton <jlayton@...nel.org>