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

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Message-ID: <20210607182335.GI18427@gate.crashing.org>
Date:   Mon, 7 Jun 2021 13:23:35 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     "Paul E. McKenney" <paulmck@...nel.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 08:27:12AM -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.

> > > 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?  ;-)

I don't know what you are referring to, sorry?

I know what you asked for: literally some way to tell the compiler to
emit a conditional branch.  If that is what you want, the only way to
make sure that is what you get is by writing exactly that in assembler.


Segher