linux-kernel - Re: perf_mmap__write_tail() and control dependencies

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Message-ID: <20150724153620.GF12569@arm.com>
Date:	Fri, 24 Jul 2015 16:36:20 +0100
From:	Will Deacon <will.deacon@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"paulus@...ba.org" <paulus@...ba.org>,
	"rostedt@...dmis.org" <rostedt@...dmis.org>
Subject: Re: perf_mmap__write_tail() and control dependencies

On Fri, Jul 24, 2015 at 04:33:16PM +0100, Peter Zijlstra wrote:
> On Fri, Jul 24, 2015 at 08:29:05AM -0700, Paul E. McKenney wrote:
> > The ring-buffer code uses control dependencies, and the shiny new
> > READ_ONCE_CTRL() is now in mainline.  I was idly curious about whether
> > the write side could use smp_store_release(), and I found this:
> > 
> > static inline void perf_mmap__write_tail(struct perf_mmap *md, u64 tail)
> > {
> > 	struct perf_event_mmap_page *pc = md->base;
> > 
> > 	/*
> > 	 * ensure all reads are done before we write the tail out.
> > 	 */
> > 	mb();
> > 	pc->data_tail = tail;
> > }
> > 
> > I see mb() rather than smp_mb().  Did I find the correct code for the
> > write side?  If so, why mb() rather than smp_mb()?  To serialize against
> > MMIO interactions with hardware counters or some such?
> 
> This is userspace, it doesn't patch itself depending on if its run on an
> SMP machine or not.

Yup, and that's why mb() expands to dmb instead of dsb in
tools/arch/arm64/include/asm/barrier.h (I see there's an XXX: comment
there asking about the difference).

Will
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