Re: [PATCH v15] x86/split_lock: Enable split lock detection by kernel

From: Peter Zijlstra
Date: Mon Jan 27 2020 - 03:04:44 EST


On Sat, Jan 25, 2020 at 01:50:03PM -0800, Luck, Tony wrote:
> On Sat, Jan 25, 2020 at 04:25:25PM -0500, Arvind Sankar wrote:
> > On Fri, Jan 24, 2020 at 06:47:27PM -0800, Luck, Tony wrote:
> > > I did find something with a new test. Applications that hit a
> > > split lock warn as expected. But if they sleep before they hit
> > > a new split lock, we get another warning. This is may be because
> > > I messed up when fixing a PeterZ typo in the untested patch.
> > > But I think there may have been bigger problems.
> > >
> > > Context switch in V14 code did:
> > >
> > > if (tifp & _TIF_SLD)
> > > switch_to_sld(prev_p);
> > >
> > > void switch_to_sld(struct task_struct *prev)
> > > {
> > > __sld_msr_set(true);
> > > clear_tsk_thread_flag(prev, TIF_SLD);
> > > }
> > >
> > > Which re-enables split lock checking for the next process to run. But
> > > mysteriously clears the TIF_SLD bit on the previous task.
> >
> > Did Peter mean to disable it only for the current timeslice and
> > re-enable it for the next time its scheduled?
>
> He's seen and commented on this thread since I made this comment. So

Yeah, I sorta don't care either way :-)

> I'll assume not. Things get really noisy on the console (even with
> the rate limit) if split lock detection is re-enabled after a context
> switch (my new test highlighted this!)

Have you found any actual bad software ? The only way I could trigger
was by explicitly writing a program to tickle it.