Re: ftrace introduces instability into kernel 2.6.27(-rc2,-rc3)

From: Steven Rostedt <rostedt@goodmis.org>
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)
Date: Tue, 19 Aug 2008 12:47:39 -0400 (EDT)	[thread overview]
Message-ID: <alpine.DEB.1.10.0808191247001.28232@gandalf.stny.rr.com> (raw)
In-Reply-To: <20080819024707.GA22659@Krystal>

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.
> 

OK, I just tried text_poke and it unfortunately fails. The problem is that 
it requires that the text you are changing is aligned and fits on one 
page. We have no control over that.

-- Steve