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

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Message-ID: <OF1519E98D.692AF083-ON42257C11.002E693E-42257C11.00317D48@il.ibm.com>
Date:	Sun, 27 Oct 2013 11:00:33 +0200
From:	Victor Kaplansky <VICTORK@...ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	anton@...ba.org, benh@...nel.crashing.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org,
	Linux PPC dev <linuxppc-dev@...abs.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	michael@...erman.id.au, Michael Neuling <mikey@...ling.org>
Subject: Re: perf events ring buffer memory barrier on powerpc

Peter Zijlstra <peterz@...radead.org> wrote on 10/25/2013 07:37:49 PM:

> I would argue for:
>
>   READ ->data_tail         READ ->data_head
>     smp_rmb()   (A)          smp_rmb()   (C)
>   WRITE $data              READ $data
>     smp_wmb()   (B)          smp_mb()   (D)
>   STORE ->data_head        WRITE ->data_tail
>
> Where A pairs with D, and B pairs with C.

1. I agree. My only concern is that architectures which do use atomic
operations
with memory barriers, will issue two consecutive barriers now, which is
sub-optimal.

2. I think the comment in "include/linux/perf_event.h" describing
"data_head" and
"data_tail" for user space need an update as well. Current version -

        /*
         * Control data for the mmap() data buffer.
         *
         * User-space reading the @data_head value should issue an rmb(),
on
         * SMP capable platforms, after reading this value -- see
         * perf_event_wakeup().
         *
         * When the mapping is PROT_WRITE the @data_tail value should be
         * written by userspace to reflect the last read data. In this case
         * the kernel will not over-write unread data.
         */
        __u64   data_head;              /* head in the data section */
        __u64   data_tail;              /* user-space written tail */

- say nothing about the need of memory barrier before "data_tail" write.

-- Victor


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