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Date: Mon, 18 Aug 2008 23:32:08 -0400 (EDT)
From: Steven Rostedt <rostedt@goodmis.org>
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To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
       Scott Wood <scottwood@freescale.com>,
       Eran Liberty <liberty@extricom.com>, linux-kernel@vger.kernel.org,
       linuxppc-dev@ozlabs.org, Steven Rostedt <srostedt@redhat.com>,
       "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: ftrace introduces instability into kernel 2.6.27(-rc2,-rc3)
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On Mon, 18 Aug 2008, Mathieu Desnoyers wrote:

> * Steven Rostedt (rostedt@goodmis.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.

-- Steve