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Date:   Sun, 6 Jun 2021 15:11:54 -0500
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Jakub Jelinek <jakub@redhat.com>,
        Alan Stern <stern@rowland.harvard.edu>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>,
        Andrea Parri <parri.andrea@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
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        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        Akira Yokosawa <akiyks@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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Subject: Re: [RFC] LKMM: Add volatile_if()
Message-ID: <20210606201154.GB18427@gate.crashing.org>
References: <20210604205600.GB4397@paulmck-ThinkPad-P17-Gen-1> <CAHk-=wgmUbU6XPHz=4NFoLMxH7j_SR-ky4sKzOBrckmvk5AJow@mail.gmail.com> <20210604214010.GD4397@paulmck-ThinkPad-P17-Gen-1> <CAHk-=wg0w5L7-iJU_kvEh9stXZoh2srRF4jKToKmSKyHv-njvA@mail.gmail.com> <20210605145739.GB1712909@rowland.harvard.edu> <20210606001418.GH4397@paulmck-ThinkPad-P17-Gen-1> <20210606012903.GA1723421@rowland.harvard.edu> <CAHk-=wgUsReyz4uFymB8mmpphuP0vQ3DktoWU_x4u6impbzphg@mail.gmail.com> <20210606185922.GF7746@tucnak> <CAHk-=wis8zq3WrEupCY6wcBeW3bB0WMOzaUkXpb-CsKuxM=6-w@mail.gmail.com>
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On Sun, Jun 06, 2021 at 12:22:44PM -0700, Linus Torvalds wrote:
> On Sun, Jun 6, 2021 at 11:59 AM Jakub Jelinek <jakub@redhat.com> wrote:
> >
> > I think just
> > #define barrier() __asm__ __volatile__("" : : "i" (__COUNTER__) : "memory")
> > should be enough
> 
> Oh, I like that. Much better.
> 
> It avoids all the issues with comments etc, and because it's not using
> __COUNTER__ as a string, it doesn't need the preprocessor games with
> double expansion either.
> 
> So yeah, that seems like a nice solution to the issue, and should make
> the barriers all unique to the compiler.

__COUNTER__ is a preprocessor thing as well, and it may not do all that
you expect.  Ex.:

===
#define fm() __COUNTER__
int gm(void) { return fm(); }
int hm(void) { return fm(); }

int fi(void) { return __COUNTER__; }
int gi(void) { return fi(); }
int hi(void) { return fi(); }
===

The macro version here works as you would hope, but the inlined one has
the same number everywhere.


Segher