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

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Message-ID: <20070716102738.GA4276@in.ibm.com>
Date:	Mon, 16 Jul 2007 15:57:38 +0530
From:	Ananth N Mavinakayanahalli <ananth@...ibm.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Christoph Hellwig <hch@...radead.org>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, prasanna@...ibm.com,
	anil.s.keshavamurthy@...el.com, davem@...emloft.net
Subject: Re: [patch 1/8] Kprobes - do not use kprobes mutex in arch code

On Sat, Jul 14, 2007 at 03:20:02PM -0400, Mathieu Desnoyers wrote:
> * 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.
> > 
> 
> Since only unregister_kprobe() calls arch_remove_kprobe(), and only
> after having removed the struct kprobe from the kprobes list (while the
> kprobes mutex is held), I wonder if there is any need to hold the
> kprobes mutex at all when calling arch_remove_kprobe(). It turns out
> that only get_insn_slot()/free_insn_slot() (which is in
> kernel/kprobes.c, but called from arch specific code) seems to really
> use protection of this mutex.

Right.

> Would it make sense to protect the kprobe_insn_pages list with a
> new kprobe_insn_mutex, nestable in the kprobe_mutex ?

Do you think it is required after your change to make kprobe_mutex
static? But yes, for architectures that don't need a arch_remove_kprobe,
the situation is a bit odd... a mutex to do nothing. IIRC, that was the
primary reason why we made the mutex visible outside of kernel/kprobes.c

Ananth
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