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Date: Fri, 31 May 2019 13:14:45 +0200
From: Peter Zijlstra <>
To: David Howells <>
Cc: Jann Horn <>, Greg KH <>,
	Al Viro <>,,
	linux-fsdevel <>,
	Linux API <>,,,
	linux-security-module <>,
	kernel list <>,
	Kees Cook <>,
	Kernel Hardening <>
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 <> 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.

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