linux-kernel - Re: perf events ring buffer memory barrier on powerpc

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Message-ID: <20131031150756.GB4067@linux.vnet.ibm.com>
Date:	Thu, 31 Oct 2013 08:07:56 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Victor Kaplansky <VICTORK@...ibm.com>,
	Anton Blanchard <anton@...ba.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux PPC dev <linuxppc-dev@...abs.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Michael Ellerman <michael@...erman.id.au>,
	Michael Neuling <mikey@...ling.org>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: perf events ring buffer memory barrier on powerpc

On Thu, Oct 31, 2013 at 10:04:57AM +0100, Peter Zijlstra wrote:
> On Wed, Oct 30, 2013 at 09:32:58PM -0700, Paul E. McKenney wrote:
> > Before C/C++11, the closest thing to such a prohibition is use of
> > volatile, for example, ACCESS_ONCE().  Even in C/C++11, you have to
> > use atomics to get anything resembing this prohibition.
> > 
> > If you just use normal variables, the compiler is within its rights
> > to transform something like the following:
> > 
> > 	if (a)
> > 		b = 1;
> > 	else
> > 		b = 42;
> > 
> > Into:
> > 
> > 	b = 42;
> > 	if (a)
> > 		b = 1;
> > 
> > Many other similar transformations are permitted.  Some are used to all
> > vector instructions to be used -- the compiler can do a write with an
> > overly wide vector instruction, then clean up the clobbered variables
> > later, if it wishes.  Again, if the variables are not marked volatile,
> > or, in C/C++11, atomic.
> 
> While I've heard you tell this story before, my mind keeps boggling how
> we've been able to use shared memory at all, all these years.
> 
> It seems to me stuff should have broken left, right and center if
> compilers were really aggressive about this.

Sometimes having stupid compilers is a good thing.  But they really are
getting more aggressive.

							Thanx, Paul

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