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Subject: Re: [PATCH] perf/x86/intel: Fix PT PMI handling
To:     Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc:     Arnaldo Carvalho de Melo <acme@redhat.com>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        Jiri Olsa <jolsa@kernel.org>,
        Vitaly Slobodskoy <vitaly.slobodskoy@intel.com>,
        stable@vger.kernel.org
References: <20191210105101.77210-1-alexander.shishkin@linux.intel.com>
From:   Alexey Budankov <alexey.budankov@linux.intel.com>
Organization: Intel Corp.
Message-ID: <aff9e887-5048-ccc2-fe03-91a5de3fdedb@linux.intel.com>
Date:   Tue, 10 Dec 2019 14:49:28 +0300
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On 10.12.2019 13:51, Alexander Shishkin wrote:
> Commit:
> 
>   ccbebba4c6bf ("perf/x86/intel/pt: Bypass PT vs. LBR exclusivity if the core supports it")
> 
> skips the PT/LBR exclusivity check on CPUs where PT and LBRs coexist, but
> also inadvertently skips the active_events bump for PT in that case, which
> is a bug. If there aren't any hardware events at the same time as PT, the
> PMI handler will ignore PT PMIs, as active_events reads zero in that case,
> resulting in the "Uhhuh" spurious NMI warning and PT data loss.
> 
> Fix this by always increasing active_events for PT events.
> 
> Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Fixes: ccbebba4c6bf ("perf/x86/intel/pt: Bypass PT vs. LBR exclusivity if the core supports it")
> Reported-by: Vitaly Slobodskoy <vitaly.slobodskoy@intel.com>
> Cc: stable@vger.kernel.org # v4.7
> ---
>  arch/x86/events/core.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)

Acked-by: Alexey Budankov <alexey.budankov@linux.intel.com>

> 
> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> index 6e3f0c18908e..5a736197dfa4 100644
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -375,7 +375,7 @@ int x86_add_exclusive(unsigned int what)
>  	 * LBR and BTS are still mutually exclusive.
>  	 */
>  	if (x86_pmu.lbr_pt_coexist && what == x86_lbr_exclusive_pt)
> -		return 0;
> +		goto out;
>  
>  	if (!atomic_inc_not_zero(&x86_pmu.lbr_exclusive[what])) {
>  		mutex_lock(&pmc_reserve_mutex);
> @@ -387,6 +387,7 @@ int x86_add_exclusive(unsigned int what)
>  		mutex_unlock(&pmc_reserve_mutex);
>  	}
>  
> +out:
>  	atomic_inc(&active_events);
>  	return 0;
>  
> @@ -397,11 +398,15 @@ int x86_add_exclusive(unsigned int what)
>  
>  void x86_del_exclusive(unsigned int what)
>  {
> +	atomic_dec(&active_events);
> +
> +	/*
> +	 * See the comment in x86_add_exclusive().
> +	 */
>  	if (x86_pmu.lbr_pt_coexist && what == x86_lbr_exclusive_pt)
>  		return;
>  
>  	atomic_dec(&x86_pmu.lbr_exclusive[what]);
> -	atomic_dec(&active_events);
>  }
>  
>  int x86_setup_perfctr(struct perf_event *event)
>