linux-kernel - Re: [perfmon] Re: [perfmon2] perfmon2 merge news

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Message-ID: <20071113222534.GC17145@one.firstfloor.org>
Date:	Tue, 13 Nov 2007 23:25:34 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Stephane Eranian <eranian@....hp.com>
Cc:	Andi Kleen <andi@...stfloor.org>, akpm@...l.org,
	Robert Richter <robert.richter@....com>, gregkh@...e.de,
	linux-kernel@...r.kernel.org, perfmon@...ali.hpl.hp.com,
	William Cohen <wcohen@...hat.com>,
	perfmon2-devel@...ts.sourceforge.net
Subject: Re: [perfmon] Re: [perfmon2] perfmon2 merge news

On Tue, Nov 13, 2007 at 02:22:34PM -0800, Stephane Eranian wrote:
> Andi,
> On Tue, Nov 13, 2007 at 10:50:56PM +0100, Andi Kleen wrote:
> > > Yes, horribly more complicated because of locking issues within perfmon.
> > > As soon as you expose a file descriptor, you need some locking to prevent
> > > multiple user threads (malicious or not) to compete to access the PMU state.
> > 
> > Why do you need the file descriptor? 
> > 
> 
> To identify your monitoring session be it system-wide (i.e., per-cpu) or per-thread.
> file descriptor allows you to use close, read, select, poll and you leverage the

Surely that could be done with a flag for each call too? Keeping file descriptors
to pass essentially a boolean seems overkill.

> existing file descriptor sharing/inheritance sematics. At the kernel level, a 
> descriptor provides all the callback necessary to make sure you clean up the perfmon
> session state on exit.

Didn't you already have a thread destructor for it?

-Andi
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