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Date: Mon, 18 Aug 2008 22:28:31 -0400 (EDT)
From: Steven Rostedt <rostedt@goodmis.org>
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To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
cc: Scott Wood <scottwood@freescale.com>, Eran Liberty <liberty@extricom.com>,
       Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
       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 Tue, 19 Aug 2008, Benjamin Herrenschmidt wrote:

> On Mon, 2008-08-18 at 14:27 -0400, Steven Rostedt wrote:
> > 
> > On Mon, 18 Aug 2008, Scott Wood wrote:
> > 
> > > Mathieu Desnoyers wrote:
> > > >         asm volatile (
> > > >                 "1: lwz         %1, 0(%2)\n"
> > > >                 "   cmpw        %1, %5\n"
> > > >                 "   bne         2f\n"
> > > >                 "   stwu        %3, 0(%2)\n"
> > > >                 "2:\n"
> > > >                 ".section .fixup, \"ax\"\n"
> > > >                 "3:     li %0, 1\n"
> > > >                 "       b 2b\n"
> > > >                 ".previous\n"
> > > >                 ".section __ex_table,\"a\"\n"
> > > >                 _ASM_ALIGN "\n"
> > > >                 _ASM_PTR "1b, 3b\n"
> > > >                 ".previous"
> > > >                 : "=r"(faulted), "=r"(replaced)
> > > >                 : "r"(ip), "r"(new),
> > > >                   "0"(faulted), "r"(old)
> > > >                 : "memory");
> > > 
> > > Some (most likely unrelated) nits in the above inline asm:
> 
> 
> Why not use __get_user/__put_user ?

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).

So are you recommending something like

        int cmd;

	if (__get_user(cmd, ip))
		goto fault;

	if (cmd != old)
		goto not_same;

	WARN_ON_ONCE(__put_user(cmd, ip));

If we did this, we could probably put this into the generic code:


	if (copy_from_user(cmd, ip, ARCH_CALL_SIZE))
		goto fault;

	if (memcmp(cmd, old, ARCH_CALL_SIZE) != 0)
		goto not_same;

	WARN_ON_ONCE(copy_to_user(cmd, ip, ARCH_CALL_SIZE));


-- Steve