linux-kernel - Re: [patch 1/8] Kprobes - do not use kprobes mutex in arch code

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Message-ID: <20070714193026.GL6975@Krystal>
Date:	Sat, 14 Jul 2007 15:30:26 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Christoph Hellwig <hch@...radead.org>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, prasanna@...ibm.com,
	ananth@...ibm.com, anil.s.keshavamurthy@...el.com,
	davem@...emloft.net
Subject: Re: [patch 1/8] Kprobes - do not use kprobes mutex in arch code

* Christoph Hellwig (hch@...radead.org) wrote:
> On Fri, Jul 13, 2007 at 09:21:34PM -0400, Mathieu Desnoyers wrote:
> > Remove the kprobes mutex from kprobes.h, since it does not belong there. Also
> > remove all use of this mutex in the architecture specific code, replacing it by
> > a proper mutex lock/unlock in the architecture agnostic code.
> 
> This is not very nice for avr32/sparc64 which have a noop arch_remove_kprobe
> and now need to take a mutex to do nothing.  Maybe you can find a nice
> way to avoid that?
> 
> Except for this issue making kprobes_mutex static to kprobes.c sounds like
> a good improvement.
> 

While we are here:

The whole check_safety() in kprobes.c seems awkward.. freezing processes
is probably costly, and the check:

if (p != current && p->state == TASK_RUNNING && p->pid != 0) {

Adds restrictions about where a probe can be safely put.. the idle
thread becomes a restriction.

I suggest disabling preemption in the int3 handler, just before
single-stepping, then reenabling it in the breakpoint handler executed
right after the single-step. A synchronize_sched() could then replace
the whole check_safety() and would never fail. The side-effect would be
to disable preemption in the single-step, it's no big deal.

Mathieu


-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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