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Message-ID: <20201006124907.GA5822@1wt.eu>
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>,
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"stern@...land.harvard.edu" <stern@...land.harvard.edu>,
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"dhowells@...hat.com" <dhowells@...hat.com>,
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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
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