linux-kernel - Re: [PATCH 00/22] add support for Clang LTO

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Message-ID: <20200701091054.GW4781@hirez.programming.kicks-ass.net>
Date:   Wed, 1 Jul 2020 11:10:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Marco Elver <elver@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Will Deacon <will@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-pci@...r.kernel.org,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>
Subject: Re: [PATCH 00/22] add support for Clang LTO

On Tue, Jun 30, 2020 at 01:30:16PM -0700, Paul E. McKenney wrote:
> On Tue, Jun 30, 2020 at 10:12:43PM +0200, Peter Zijlstra wrote:

> > I'm not convinced C11 memory_order_consume would actually work for us,
> > even if it would work. That is, given:
> > 
> >   https://lore.kernel.org/lkml/20150520005510.GA23559@linux.vnet.ibm.com/
> > 
> > only pointers can have consume, but like I pointed out, we have code
> > that relies on dependent loads from integers.
> 
> I agree that C11 memory_order_consume is not normally what we want,
> given that it is universally promoted to memory_order_acquire.
> 
> However, dependent loads from integers are, if anything, more difficult
> to defend from the compiler than are control dependencies.  This applies
> doubly to integers that are used to index two-element arrays, in which
> case you are just asking the compiler to destroy your dependent loads
> by converting them into control dependencies.

Yes, I'm aware. However, as you might know, I'm firmly in the 'C is a
glorified assembler' camp (as I expect most actual OS people are, out of
necessity if nothing else) and if I wanted a control dependency I
would've bloody well written one.

I think an optimizing compiler is awesome, but only in so far as that
optimization is actually helpful -- and yes, I just stepped into a giant
twilight zone there. That is, any optimization that has _any_
controversy should be controllable (like -fno-strict-overflow
-fno-strict-aliasing) and I'd very much like the same here.

In a larger context, I still think that eliminating speculative stores
is both necessary and sufficient to avoid out-of-thin-air. So I'd also
love to get some control on that.