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

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Message-ID: <20200701150512.GH4817@hirez.programming.kicks-ass.net>
Date:   Wed, 1 Jul 2020 17:05:12 +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 Wed, Jul 01, 2020 at 07:06:54AM -0700, Paul E. McKenney wrote:

> The current state in the C++ committee is that marking variables
> carrying dependencies is the way forward.  This is of course not what
> the Linux kernel community does, but it should not be hard to have a
> -fall-variables-dependent or some such that causes all variables to be
> treated as if they were marked.  Though I was hoping for only pointers.
> Are they -sure- that they -absolutely- need to carry dependencies
> through integers???

What's 'need'? :-)

I'm thinking __ktime_get_fast_ns() is better off with a dependent load
than it is with an extra smp_rmb().

Yes we can stick an smp_rmb() in there, but I don't like it. Like I
wrote earlier, if I wanted a control dependency, I'd have written one.