On Thu, 27 Feb 2014 18:19:37 +0100 Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote: > On Thu, Feb 27, 2014 at 12:00:14PM -0500, Steven Rostedt wrote: > > 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(). > > Ah ok. My understanding was indeed that it doesn't ignore the ftrace trap, > but I thought the consequence was that we return immediately from the trap > handler. I'll add my above cpu race diagram (is that what we call it?). That should make this change more understandable. > Ok but what I meant is to do this instead: > > fail_update: > probe_kernel_write((void *)ip, &old_code[0], 1); > + run_sync() > goto out; > > Because with the current patch we also call run_sync() on add_break() failure. Ah ok (my turn to understand). Yeah, if the add_break() fails, then we don't need to do the run_sync(). But this is just for now, to prevent the add_update_code() error from crashing. I have more patches that clean this up further. But they are for 3.15. -- Steve -- 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