On Thu, 27 Feb 2014 17:37:32 +0100 Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote: > On Thu, Feb 27, 2014 at 10:46:18AM -0500, Steven Rostedt wrote: > > [Request for Ack] > > > > From: Petr Mladek <pmladek@xxxxxxx> > > > > 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@xxxxxxx > > > > Fixes: 8a4d0a687a5 "ftrace: Use breakpoint method to update ftrace caller" > > Cc: stable@xxxxxxxxxxxxxxx # 3.5+ > > Signed-off-by: Petr Mladek <pmladek@xxxxxxx> > > Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx> > > --- > > 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@xxxxxxxxx> > > > -- > > 1.8.5.3 > > > > -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html