linux-kernel - Re: [Patch v3 3/7] perf/x86: Check if cpuc->events[*] pointer exists before accessing it

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Message-ID: <20250821133514.GP4067720@noisy.programming.kicks-ass.net>
Date: Thu, 21 Aug 2025 15:35:14 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Dapeng Mi <dapeng1.mi@...ux.intel.com>
Cc: Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>, Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Kan Liang <kan.liang@...ux.intel.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Eranian Stephane <eranian@...gle.com>, linux-kernel@...r.kernel.org,
	linux-perf-users@...r.kernel.org, Dapeng Mi <dapeng1.mi@...el.com>,
	kernel test robot <oliver.sang@...el.com>
Subject: Re: [Patch v3 3/7] perf/x86: Check if cpuc->events[*] pointer exists
 before accessing it

On Wed, Aug 20, 2025 at 10:30:28AM +0800, Dapeng Mi wrote:
> When intel_pmu_drain_pebs_icl() is called to drain PEBS records, the
> perf_event_overflow() could be called to process the last PEBS record.
> 
> While perf_event_overflow() could trigger the interrupt throttle and
> stop all events of the group, like what the below call-chain shows.
> 
> perf_event_overflow()
>   -> __perf_event_overflow()
>     ->__perf_event_account_interrupt()
>       -> perf_event_throttle_group()
>         -> perf_event_throttle()
>           -> event->pmu->stop()
>             -> x86_pmu_stop()
> 
> The side effect of stopping the events is that all corresponding event
> pointers in cpuc->events[] array are cleared to NULL.
> 
> Assume there are two PEBS events (event a and event b) in a group. When
> intel_pmu_drain_pebs_icl() calls perf_event_overflow() to process the
> last PEBS record of PEBS event a, interrupt throttle is triggered and
> all pointers of event a and event b are cleared to NULL. Then
> intel_pmu_drain_pebs_icl() tries to process the last PEBS record of
> event b and encounters NULL pointer access.
> 
> Since the left PEBS records have been processed when stopping the event,
> check and skip to process the last PEBS record if cpuc->events[*] is
> NULL.
> 
> Reported-by: kernel test robot <oliver.sang@...el.com>
> Closes: https://lore.kernel.org/oe-lkp/202507042103.a15d2923-lkp@intel.com
> Fixes: 9734e25fbf5a ("perf: Fix the throttle logic for a group")
> Signed-off-by: Dapeng Mi <dapeng1.mi@...ux.intel.com>
> Tested-by: kernel test robot <oliver.sang@...el.com>
> ---
>  arch/x86/events/intel/ds.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
> index c0b7ac1c7594..dcf29c099ad2 100644
> --- a/arch/x86/events/intel/ds.c
> +++ b/arch/x86/events/intel/ds.c
> @@ -2663,6 +2663,16 @@ static void intel_pmu_drain_pebs_icl(struct pt_regs *iregs, struct perf_sample_d
>  			continue;
>  
>  		event = cpuc->events[bit];
> +		/*
> +		 * perf_event_overflow() called by below __intel_pmu_pebs_last_event()
> +		 * could trigger interrupt throttle and clear all event pointers of the
> +		 * group in cpuc->events[] to NULL. So need to re-check if cpuc->events[*]
> +		 * is NULL, if so it indicates the event has been throttled (stopped) and
> +		 * the corresponding last PEBS records have been processed in stopping
> +		 * event, don't need to process it again.
> +		 */
> +		if (!event)
> +			continue;
>  
>  		__intel_pmu_pebs_last_event(event, iregs, regs, data, last[bit],
>  					    counts[bit], setup_pebs_adaptive_sample_data);


So if this is due to __intel_pmu_pebs_last_event() calling into
perf_event_overflow(); then isn't intel_pmu_drain_pebs_nhm() similarly
affected?

And worse, the _nhm() version would loose all events for that counter,
not just the last.

I'm really thinking this isn't the right thing to do.


How about we audit the entirety of arch/x86/events/ for cpuc->events[]
usage and see if we can get away with changing x86_pmu_stop() to simply
not clearing that field.

Or perhaps move the setting and clearing into x86_pmu_{add,del}() rather
than x86_pmu_{start,stop}(). After all, the latter don't affect the
counter placement, they just stop/start the event.