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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun, 6 Jun 2021 11:48:32 -0700
Message-ID: <CAHk-=wjEHbGifWgA+04Y4_m43s-o+3bXpL5qPQL3ECg+86XuLg@mail.gmail.com>
Subject: Re: [RFC] LKMM: Add volatile_if()
To:     Segher Boessenkool <segher@kernel.crashing.org>
Cc:     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>,
        Nick Piggin <npiggin@gmail.com>,
        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>,
        linux-toolchains@vger.kernel.org,
        linux-arch <linux-arch@vger.kernel.org>
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On Sun, Jun 6, 2021 at 11:43 AM Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> You truly should have written a branch in tthe asm if you truly wanted
> a branch instruction.

That's exactly what I don't want to do, and what the original patch by
PeterZ did.

Why?

Because then we need to write that stupid pointless branch for every
single architecture.

And to work well, it needs "asm goto", which is so recent that a lot
of compilers don't support it (thank God for clang dragging gcc
kicking and screaming to implement it at all - I'd asked for it over a
decade ago).

So you get bad code generation in a lot of cases, which entirely
obviates the _point_ of this all - which is that we can avoid an
expensive operation (a memory barrier) by just doing clever code
generation.

So if we can't get the clever code generation, it's all pretty much
moot, imnsho.

A working barrier "just fixes it".

I suspect the best we can do is to just work around the gcc badness
with that __COUNTER__ trick of mine. The lack of a reliable comment
character is the biggest issue with that trick.

                 Linus