linux-kernel - Re: [PATCH 1/7] General notification queue with user mmap()'able ring buffer

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Message-ID: <20190531111445.GO2677@hirez.programming.kicks-ass.net>
Date:   Fri, 31 May 2019 13:14:45 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Jann Horn <jannh@...gle.com>, Greg KH <gregkh@...uxfoundation.org>,
        Al Viro <viro@...iv.linux.org.uk>, raven@...maw.net,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        linux-block@...r.kernel.org, keyrings@...r.kernel.org,
        linux-security-module <linux-security-module@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 1/7] General notification queue with user mmap()'able
 ring buffer

On Wed, May 29, 2019 at 10:02:43PM +0100, David Howells wrote:
> Jann Horn <jannh@...gle.com> wrote:
> 
> > Does this mean that refcount_read() isn't sufficient for what you want
> > to do with tracing (because for some reason you actually need to know
> > the values atomically at the time of increment/decrement)?
> 
> Correct.  There's a gap and if an interrupt or something occurs, it's
> sufficiently big for the refcount trace to go weird.
> 
> I've seen it in afs/rxrpc where the incoming network packets that are part of
> the rxrpc call flow disrupt the refcounts noted in trace lines.

Can you re-iterate the exact problem? I konw we talked about this in the
past, but I seem to have misplaced those memories :/

FWIW I agree that kref is useless fluff, but I've long ago given up on
that fight.