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Date: Thu, 27 Feb 2014 12:00:14 -0500
From: Steven Rostedt <rostedt@goodmis.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        stable@vger.kernel.org, Petr Mladek <pmladek@suse.cz>
Subject: Re: [RFA][PATCH 2/5] ftrace/x86: One more missing sync after fixup
 of function modification failure
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On Thu, 27 Feb 2014 17:37:32 +0100
Frederic Weisbecker <fweisbec@gmail.com> wrote:

> On Thu, Feb 27, 2014 at 10:46:18AM -0500, Steven Rostedt wrote:
> > [Request for Ack]
> > 
> > From: Petr Mladek <pmladek@suse.cz>
> > 
> > If a failure occurs while modifying ftrace function, it bails out and will
> > remove the tracepoints to be back to what the code originally was.
> > 
> > There is missing the final sync run across the CPUs after the fix up is done
> > and before the ftrace int3 handler flag is reset.
> 
> So IIUC the risk is that other CPUs may spuriously ignore non-ftrace traps if we don't sync the
> other cores after reverting the int3 before decrementing the modifying_ftrace_code counter?

Actually, the bug is that they will not ignore the ftrace traps after
we decrement modifying_ftrace_code counter. Here's the race:

	CPU0				CPU1
	----				----
  remove_breakpoint();
  modifying_ftrace_code = 0;

				[still sees breakpoint]
				<takes trap>
				[sees modifying_ftrace_code as zero]
				[no breakpoint handler]
				[goto failed case]
				[trap exception - kernel breakpoint, no
				 handler]
				BUG()


Even if we had a smp_wmb() after removing the breakpoint and clearing
the modifying_ftrace_code, we still need the smp_rmb() on the other
CPUS. The run_sync() does a IPI on all CPUs doing the smp_rmb().

> 
> > 
> > Link: http://lkml.kernel.org/r/1393258342-29978-2-git-send-email-pmladek@suse.cz
> > 
> > Fixes: 8a4d0a687a5 "ftrace: Use breakpoint method to update ftrace caller"
> > Cc: stable@vger.kernel.org # 3.5+
> > Signed-off-by: Petr Mladek <pmladek@suse.cz>
> > Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> > ---
> >  arch/x86/kernel/ftrace.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
> > index 6b566c8..69885e2 100644
> > --- a/arch/x86/kernel/ftrace.c
> > +++ b/arch/x86/kernel/ftrace.c
> > @@ -660,8 +660,8 @@ ftrace_modify_code(unsigned long ip, unsigned const char *old_code,
> >  		ret = -EPERM;
> >  		goto out;
> >  	}
> > -	run_sync();
> >   out:
> > +	run_sync();
> >  	return ret;
> >  
> >   fail_update:
> 
> This could be further optimized by rather calling run_sync() in the end of the
> fail_update block (after the probe_kernel_write revert) otherwise even failure on
> setting the break will result in run_sync(), which doesn't appear to be needed. But
> that's really just nitpicking as it's a rare failure codepath and shouldn't hurt.

No, the run_sync() must be done after removing the breakpoint. Again,
we don't want one of these breakpoints to be called on another CPU and
then see modifying_ftrace_code as zero. That is bad. The final
run_sync() is required.

I think I'll update the change log to include my race flow graph from
above.

-- Steve


> 
> In any case, the fix looks correct.
> 
> Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
> 
> > -- 
> > 1.8.5.3
> > 
> >