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Date:	Mon, 18 Aug 2008 23:36:18 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Scott Wood <scottwood@...escale.com>,
	Eran Liberty <liberty@...ricom.com>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...abs.org,
	Steven Rostedt <srostedt@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: ftrace introduces instability into kernel 2.6.27(-rc2,-rc3)

* Steven Rostedt (rostedt@...dmis.org) wrote:
> 
> On Mon, 18 Aug 2008, Mathieu Desnoyers wrote:
> 
> > * Steven Rostedt (rostedt@...dmis.org) wrote:
> > > 
> > > On Tue, 19 Aug 2008, Benjamin Herrenschmidt wrote:
> > > 
> > > > 
> > > > > Hmm, this was originally copied from x86, where we did a cmpxchg, but that 
> > > > > is probably not needed since all of this is done in kstop_machine. Also, 
> > > > > only the "get" is needed. If we don't fault there, we wont fault on the 
> > > > > put (unless we have permissions wrong, and that would be a bug).
> > > > 
> > > > Would it ? How do we make sure the kernel text is mapped writeable ?
> > > 
> > > We map it writeable if FTRACE is enabled.
> > > 
> > 
> > Argh. See text_poke(). It's there exactly for this purpose on x86.
> 
> Ouch, I just did. text_poke is quite heavy. It would be interesting to see 
> that performed on 20,000 locations at one time. I could play with it, but 
> I'm a bit nervous.
> 

It's alread used to modify the LOCK prefixes in alternative.c and did
not seem to be too slow for that.. it should therefore be ok.

Mathieu

> -- Steve

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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