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

[Peter Zijlstra <peterz@infradead.org>]

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?

That was the initial approach, yes. I was thinking it might help find
multiple spots in bad programs.

And as I said; I used perf on my desktop and couldn't find a single bad
program, so I'm not actually expecting this to trigger much.