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

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Date:   Mon, 27 Jan 2020 09:02:32 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     "Luck, Tony" <tony.luck@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v15] x86/split_lock: Enable split lock detection by kernel

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.