linux-kernel - Re: Control Dependencies vs C Compilers

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Date:   Tue, 6 Oct 2020 14:49:07 +0200
From:   Willy Tarreau <w@....eu>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     "'Peter Zijlstra'" <peterz@...radead.org>,
        "linux-toolchains@...r.kernel.org" <linux-toolchains@...r.kernel.org>,
        Will Deacon <will@...nel.org>,
        Paul McKenney <paulmck@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stern@...land.harvard.edu" <stern@...land.harvard.edu>,
        "parri.andrea@...il.com" <parri.andrea@...il.com>,
        "boqun.feng@...il.com" <boqun.feng@...il.com>,
        "npiggin@...il.com" <npiggin@...il.com>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        "j.alglave@....ac.uk" <j.alglave@....ac.uk>,
        "luc.maranget@...ia.fr" <luc.maranget@...ia.fr>,
        "akiyks@...il.com" <akiyks@...il.com>,
        "dlustig@...dia.com" <dlustig@...dia.com>,
        "joel@...lfernandes.org" <joel@...lfernandes.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>
Subject: Re: Control Dependencies vs C Compilers

On Tue, Oct 06, 2020 at 12:37:06PM +0000, David Laight wrote:
> > Our Documentation/memory-barriers.txt has a Control Dependencies section
> > (which I shall not replicate here for brevity) which lists a number of
> > caveats. But in general the work-around we use is:
> > 
> > 	x = READ_ONCE(*foo);
> > 	if (x > 42)
> > 		WRITE_ONCE(*bar, 1);
> 
> An alternative is to 'persuade' the compiler that
> any 'tracked' value for a local variable is invalid.
> Rather like the way that barrier() 'invalidates' memory.
> So you generate:
> 
> 	x = *foo
> 	asm ("" : "+r" (x));
> 	if (x > 42)
> 		*bar = 1;
> 
> Since the "+r" constraint indicates that the value of 'x'
> might have changed it can't optimise based on any
> presumed old value.
> (Unless it looks inside the asm opcodes...)

I'm using exactly this in userland to prevent the compiler from guessing
what I'm doing with a variable, and it's also useful sometimes to shut up
certain warnings when I know a condition is satisfied but can hardly be
expressed in a way to please the compiler. Overall I find that it's no
big deal and forces the developer to think twice before doing it, which
is probably a good thing in general.

Willy