On Thu, 27 Feb 2014 17:37:32 +0100
Frederic Weisbecker <fweis...@gmail.com> wrote:
> On Thu, Feb 27, 2014 at 10:46:18AM -0500, Steven Rostedt wrote:
> > [Request for Ack]
> >
> > From: Petr Mladek <pmla...@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-pmla...@suse.cz
> >
> > Fixes: 8a4d0a687a5 "ftrace: Use breakpoint method to update ftrace caller"
> > Cc: sta...@vger.kernel.org # 3.5+
> > Signed-off-by: Petr Mladek <pmla...@suse.cz>
> > Signed-off-by: Steven Rostedt <rost...@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 <fweis...@gmail.com>
>
> > --
> > 1.8.5.3
> >
> >
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