linux-kernel - Re: [PATCH] Linux Kernel Markers 0.13 for 2.6.17

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Message-ID: <y0mhcyue7ch.fsf@ton.toronto.redhat.com>
Date:	26 Sep 2006 15:49:34 -0400
From:	fche@...hat.com (Frank Ch. Eigler)
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Martin Bligh <mbligh@...gle.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	prasanna@...ibm.com, Andrew Morton <akpm@...l.org>,
	Ingo Molnar <mingo@...e.hu>, Paul Mundt <lethal@...ux-sh.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Jes Sorensen <jes@....com>, Tom Zanussi <zanussi@...ibm.com>,
	Richard J Moore <richardj_moore@...ibm.com>,
	Michel Dagenais <michel.dagenais@...ymtl.ca>,
	Christoph Hellwig <hch@...radead.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	William Cohen <wcohen@...hat.com>, ltt-dev@...fik.org,
	systemtap@...rces.redhat.com, Alan Cox <alan@...rguk.ukuu.org.uk>,
	Karim Yaghmour <karim@...rsys.com>,
	Pavel Machek <pavel@...e.cz>, Joe Perches <joe@...ches.com>,
	"Randy.Dunlap" <rdunlap@...otime.net>,
	"Jose R. Santos" <jrs@...ibm.com>
Subject: Re: [PATCH] Linux Kernel Markers 0.13 for 2.6.17

Mathieu Desnoyers <compudj@...stal.dyndns.org> writes:

> [...]
> > Yep, that looks reasonable.  Though you could just directly test a 
> > per-marker enable flag, rather than using "condition"...
> [...]
> I am not sure I understand your suggestion correctly.. do you mean having
> a per-marker flag that would be loaded and tested at every marker site ?

I gather that one reason for working so hard with the inline assembly
is a race condition problem with the plain STAP_MARK style of marker
disconnection:

        if (pointer) (*pointer)(args ...);

Granted, but this problem could almost certainly be dealt with simpler
than that.  How about a compxchg or other atomic-fetch of the static
pointer with a local variable?  That should solve the worry of an
(*NULL) call.

If we then become concerned with a valid pointer become obsolete (the
probe handler function wanting to unload), we might be able to use
some RCU-type deferral mechanism and/or preempt controls to ensure
that this does not happen.

- FChE
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