linux-kernel - Re: [tip:perf/core] tools/perf: Add required memory barriers

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Date:	Wed, 6 Nov 2013 15:44:56 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vince Weaver <vince@...ter.net>
Cc:	mingo@...nel.org, hpa@...or.com, anton@...ba.org,
	mathieu.desnoyers@...ymtl.ca, linux-kernel@...r.kernel.org,
	michael@...erman.id.au, paulmck@...ux.vnet.ibm.com,
	benh@...nel.crashing.org, fweisbec@...il.com, VICTORK@...ibm.com,
	tglx@...utronix.de, oleg@...hat.com, mikey@...ling.org,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perf/core] tools/perf: Add required memory barriers

On Wed, Nov 06, 2013 at 03:00:11PM +0100, Peter Zijlstra wrote:
> On Wed, Nov 06, 2013 at 08:50:47AM -0500, Vince Weaver wrote:
> > On Wed, 6 Nov 2013, tip-bot for Peter Zijlstra wrote:
> > 
> > > +++ b/tools/perf/util/evlist.h
> > > @@ -177,7 +177,7 @@ int perf_evlist__strerror_open(struct perf_evlist *evlist, int err, char *buf, s
> > >  static inline unsigned int perf_mmap__read_head(struct perf_mmap *mm)
> > >  {
> > >  	struct perf_event_mmap_page *pc = mm->base;
> > > -	int head = pc->data_head;
> > > +	int head = ACCESS_ONCE(pc->data_head);
> > >  	rmb();
> > >  	return head;
> > 
> > so is this ACCESS_ONCE required now for proper access to the mmap buffer?
> 
> Pretty much; otherwise your C compiler is allowed to mess it up.

long head = ((__atomic long)pc->data_head).load(memory_order_acquire);

coupled with:

((__atomic long)pc->data_tail).store(tail, memory_order_release);

might be the 'right' and proper C11 incantations to avoid having to
touch kernel macros; but would obviously require a recent compiler.

Barring that, I think we're stuck with:

long head = ACCESS_ONCE(pc->data_head);
smp_rmb();

...

smp_mb();
pc->data_tail = tail;

And using the right asm goo for the barriers. That said, all these asm
barriers should include a compiler barriers (memory clobber) which
_should_ avoid the worst compiler trickery -- although I don't think it
completely obviates the need for ACCESS_ONCE() -- uncertain there.
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