linux-kernel - Re: [RFC] LKMM: Add volatile

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Message-ID: <20210607152712.GR4397@paulmck-ThinkPad-P17-Gen-1>
Date:   Mon, 7 Jun 2021 08:27:12 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Andrea Parri <parri.andrea@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Nick Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-toolchains@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [RFC] LKMM: Add volatile_if()

On Mon, Jun 07, 2021 at 09:12:42AM -0500, Segher Boessenkool wrote:
> 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.

Hmmm...  What do you see Peter asking for that is different than what
I am asking for?  ;-)

							Thanx, Paul