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From:   Arvind Sankar <nivedita@alum.mit.edu>
Date:   Sat, 25 Jan 2020 16:25:25 -0500
To:     "Luck, Tony" <tony.luck@intel.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Arvind Sankar <nivedita@alum.mit.edu>,
        "Christopherson, Sean J" <sean.j.christopherson@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        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
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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?

> 
> I think we need to consider TIF_SLD state of both previous and next
> process when deciding what to do with the MSR. Three cases:
> 
> 1) If they are both the same, leave the MSR alone it is (probably) right (modulo
>    the other thread having messed with it).
> 2) Next process has _TIF_SLD set ... disable checking
> 3) Next process doesn't have _TIF_SLD set ... enable checking
> 
> So please look closely at the new version of switch_to_sld() which is
> now called unconditonally on every switch ... but commonly will do
> nothing.
...
> +	/*
> +	 * Disable the split lock detection for this task so it can make
> +	 * progress and set TIF_SLD so the detection is reenabled via
> +	 * switch_to_sld() when the task is scheduled out.
> +	 */
> +	__sld_msr_set(false);
> +	set_tsk_thread_flag(current, TIF_SLD);
> +	return true;
> +}
> +
> +void switch_to_sld(struct task_struct *prev, struct task_struct *next)
> +{
> +	bool prevflag = test_tsk_thread_flag(prev, TIF_SLD);
> +	bool nextflag = test_tsk_thread_flag(next, TIF_SLD);
> +
> +	/*
> +	 * If we are switching between tasks that have the same
> +	 * need for split lock checking, then the MSR is (probably)
> +	 * right (modulo the other thread messing with it.
> +	 * Otherwise look at whether the new task needs split
> +	 * lock enabled.
> +	 */
> +	if (prevflag != nextflag)
> +		__sld_msr_set(nextflag);
> +}

I might be missing something but shouldnt this be !nextflag given the
flag being unset is when the task wants sld?