linux-kernel - Re: [PATCH v5 00/18] Rework READ

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Date:   Wed, 13 May 2020 13:40:22 +0100
From:   Will Deacon <will@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Marco Elver <elver@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v5 00/18] Rework READ_ONCE() to improve codegen

On Wed, May 13, 2020 at 02:32:43PM +0200, Peter Zijlstra wrote:
> On Wed, May 13, 2020 at 01:48:41PM +0200, Marco Elver wrote:
> 
> > Disabling most instrumentation for arch/x86 is reasonable. Also fine
> > with the __READ_ONCE/__WRITE_ONCE changes (your improved
> > compiler-friendlier version).
> > 
> > We likely can't have both: still instrument __READ_ONCE/__WRITE_ONCE
> > (as Will suggested) *and* avoid double-instrumentation in arch_atomic.
> > If most use-cases of __READ_ONCE/__WRITE_ONCE are likely to use
> > data_race() or KCSAN_SANITIZE := n anyway, I'd say it's reasonable for
> > now.

I agree that Peter's patch is the right thing to do for now. I was hoping we
could instrument __{READ,WRITE}_ONCE(), but that we before I realised that
__no_sanitize_or_inline doesn't seem to do anything.

> Right, if/when people want sanitize crud enabled for x86 I need
> something that:
> 
>  - can mark a function 'no_sanitize' and all code that gets inlined into
>    that function must automagically also not get sanitized. ie. make
>    inline work like macros (again).
> 
> And optionally:
> 
>  - can mark a function explicitly 'sanitize', and only when an explicit
>    sanitize and no_sanitize mix in inlining give the current
>    incompatible attribute splat.
> 
> That way we can have the noinstr function attribute imply no_sanitize
> and frob the DEFINE_IDTENTRY*() macros to use (a new) sanitize_or_inline
> helper instead of __always_inline for __##func().

Sounds like a good plan to me, assuming the compiler folks are onboard.
In the meantime, can we kill __no_sanitize_or_inline and put it back to
the old __no_kasan_or_inline, which I think simplifies compiler.h and
doesn't mislead people into using the function annotation to avoid KCSAN?

READ_ONCE_NOCHECK should also probably be READ_ONCE_NOKASAN, but I
appreciate that's a noisier change.

Will