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Date:   Fri, 22 Nov 2019 16:30:56 -0800
From:   "Luck, Tony" <tony.luck@intel.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Ingo Molnar <mingo@kernel.org>, Fenghua Yu <fenghua.yu@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        H Peter Anvin <hpa@zytor.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Ravi V Shankar <ravi.v.shankar@intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>
Subject: Re: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by
 kernel parameter
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On Fri, Nov 22, 2019 at 04:27:15PM +0100, Peter Zijlstra wrote:

This all looks dubious on an HT system .... three snips
from your patch:

> +static bool __sld_msr_set(bool on)
> +{
> +	u64 test_ctrl_val;
> +
> +	if (rdmsrl_safe(MSR_TEST_CTRL, test_ctrl_val))
> +		return false;
> +
> +	if (on)
> +		test_ctrl_val |= MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
> +	else
> +		test_ctrl_val &= ~MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
> +
> +	if (wrmsrl_safe(MSR_TEST_CTRL, test_ctrl_val))
> +		return false;
> +
> +	return true;
> +}

> +void switch_sld(struct task_struct *prev)
> +{
> +	__sld_set_msr(true);
> +	clear_tsk_thread_flag(current, TIF_CLD);
> +}

> @@ -654,6 +654,9 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p)
>  		/* Enforce MSR update to ensure consistent state */
>  		__speculation_ctrl_update(~tifn, tifn);
>  	}
> +
> +	if (tifp & _TIF_SLD)
> +		switch_sld(prev_p);
>  }

Don't you have some horrible races between the two logical
processors on the same core as they both try to set/clear the
MSR that is shared at the core level?

-Tony