linux-kernel - Re: [PATCH 1/5] perf: Disallow mmap() on per-task inherited events

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Message-ID: <20100525005528.GB30395@drongo>
Date:	Tue, 25 May 2010 10:55:28 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] perf: Disallow mmap() on per-task inherited events

On Tue, May 18, 2010 at 03:32:59PM +0200, Peter Zijlstra wrote:

> Since we now have working per-task-per-cpu events for a while,
> disallow mmap() on per-task inherited events. Those things were
> a performance problem anyway, and doing away with it allows
> us to optimize the buffer somewhat by assuming there is only a single
> writer.

This also disallows user-space access to hardware counter for this
event -- which is arguably OK, since doing the userspace read would
give a different answer to read() on the event fd, as the read() sums
up all the child counters for us.  Nevertheless, I think this
side-effect is worth mentioning.

Paul.
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