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

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Message-ID: <20210605031403.GA1701165@rowland.harvard.edu>
Date:   Fri, 4 Jun 2021 23:14:03 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        "Paul E. McKenney" <paulmck@...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 Fri, Jun 04, 2021 at 12:09:26PM -0700, Linus Torvalds wrote:
> Side note: it is worth noting that my version of "volatile_if()" has
> an added little quirk: it _ONLY_ orders the stuff inside the
> if-statement.
> 
> I do think it's worth not adding new special cases (especially that
> "asm goto" hack that will generate worse code than the compiler could
> do), but it means that
> 
>     x = READ_ONCE(ptr);
>     volatile_if (x > 0)
>         WRITE_ONCE(*z, 42);
> 
> has an ordering, but if you write it as
> 
>     x = READ_ONCE(ptr);
>     volatile_if (x <= 0)
>         return;
>     WRITE_ONCE(*z, 42);
> 
> then I could in theory see teh compiler doing that WRITE_ONCE() as
> some kind of non-control dependency.

This may be a minor point, but can that loophole be closed as follows?

define volatile_if(x) \
	if ((({ _Bool __x = (x); BUILD_BUG_ON(__builtin_constant_p(__x)); __x; }) && \
		({ barrier(); 1; })) || ({ barrier(); 0; }))

(It's now a little later at night than when I usually think about this 
sort of thing, so my brain isn't firing on all its cylinders.  Forgive 
me if this is a dumb question.)

Alan