Date: Tue, 29 Oct 2013 11:35:26 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Victor Kaplansky <VICTORK@il.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>, Anton Blanchard <anton@samba.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux PPC dev <linuxppc-dev@ozlabs.org>,
        Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
        Michael Ellerman <michael@ellerman.id.au>,
        Michael Neuling <mikey@neuling.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: perf events ring buffer memory barrier on powerpc
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On Tue, Oct 29, 2013 at 11:30:57AM +0100, Peter Zijlstra wrote:
> @@ -154,9 +175,11 @@ int perf_output_begin(struct perf_output
>  		 * Userspace could choose to issue a mb() before updating the
>  		 * tail pointer. So that all reads will be completed before the
>  		 * write is issued.
> +		 *
> +		 * See perf_output_put_handle().
>  		 */
>  		tail = ACCESS_ONCE(rb->user_page->data_tail);
> -		smp_rmb();
> +		smp_mb();
>  		offset = head = local_read(&rb->head);
>  		head += size;
>  		if (unlikely(!perf_output_space(rb, tail, offset, head)))

That said; it would be very nice to be able to remove this barrier. This
is in every event write path :/
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