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Date:   Mon, 7 Jun 2021 09:12:42 -0500
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Alan Stern <stern@rowland.harvard.edu>,
        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>
Subject: Re: [RFC] LKMM: Add volatile_if()
Message-ID: <20210607141242.GD18427@gate.crashing.org>
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On Sun, Jun 06, 2021 at 04:37:29PM -0700, Paul E. McKenney wrote:
> > > The barrier() thing can work - all we need to do is to simply make it
> > > impossible for gcc to validly create anything but a conditional
> > > branch.
> > 
> > And the only foolproof way of doing that is by writing a branch.

[ ... ]

> > I am saying that if you depend on that some C code you write will result
> > in some particular machine code, without actually *forcing* the compiler
> > to output that exact machine code, then you will be disappointed.  Maybe
> > not today, and maybe it will take years, if you are lucky.
> > 
> > (s/forcing/instructing/ of course, compilers have feelings too!)
> 
> OK, I will bite...
> 
> What would you suggest as a way of instructing the compiler to emit the
> conditional branch that we are looking for?

You write it in the assembler code.

Yes, it sucks.  But it is the only way to get a branch if you really
want one.  Now, you do not really need one here anyway, so there may be
some other way to satisfy the actual requirements.


Segher