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

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Message-ID: <20131030191141.GR2490@laptop.programming.kicks-ass.net>
Date:	Wed, 30 Oct 2013 20:11:41 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Victor Kaplansky <VICTORK@...ibm.com>
Cc:	paulmck@...ux.vnet.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 Wed, Oct 30, 2013 at 07:29:30PM +0100, Peter Zijlstra wrote:
> +	page_shift = PAGE_SHIFT + page_order(rb);
> +
> +	handle->page = (offset >> page_shift) & (rb->nr_pages - 1);
> +
> +	offset &= page_shift - 1;

offset &= (1UL << page_shift) - 1;

Weird that it even appeared to work.. /me wonders if he even booted the
right kernel.

> +
> +	handle->addr = rb->data_pages[handle->page] + offset;
> +	handle->size = (1 << page_shift) - offset;
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