Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 27 Jan 2020 09:02:32 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     "Luck, Tony" <tony.luck@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Christopherson, Sean J" <sean.j.christopherson@intel.com>,
        Ingo Molnar <mingo@kernel.org>,
        "Yu, Fenghua" <fenghua.yu@intel.com>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        H Peter Anvin <hpa@zytor.com>,
        "Raj, Ashok" <ashok.raj@intel.com>,
        "Shankar, Ravi V" <ravi.v.shankar@intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>
Subject: Re: [PATCH v15] x86/split_lock: Enable split lock detection by kernel
Message-ID: <20200127080232.GF14914@hirez.programming.kicks-ass.net>
References: <20200122185514.GA16010@agluck-desk2.amr.corp.intel.com>
 <20200122224245.GA2331824@rani.riverdale.lan>
 <3908561D78D1C84285E8C5FCA982C28F7F54887A@ORSMSX114.amr.corp.intel.com>
 <20200123004507.GA2403906@rani.riverdale.lan>
 <20200123035359.GA23659@agluck-desk2.amr.corp.intel.com>
 <20200123044514.GA2453000@rani.riverdale.lan>
 <20200123231652.GA4457@agluck-desk2.amr.corp.intel.com>
 <87h80kmta4.fsf@nanos.tec.linutronix.de>
 <20200125024727.GA32483@agluck-desk2.amr.corp.intel.com>
 <20200125212524.GA538225@rani.riverdale.lan>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200125212524.GA538225@rani.riverdale.lan>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.