Re: [PATCH] perf/arm-dmc620: Fix lockdep assert in ->event_init()

From: Robin Murphy <robin.murphy@arm.com>
To: Will Deacon <will@kernel.org>, Namhyung Kim <namhyung@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	Greg Thelen <gthelen@google.com>,
	Tuan Phan <tuanphan@os.amperecomputing.com>
Subject: Re: [PATCH] perf/arm-dmc620: Fix lockdep assert in ->event_init()
Date: Fri, 17 May 2024 13:16:16 +0100	[thread overview]
Message-ID: <e5497be9-85c7-42ac-925d-194b690a0eac@arm.com> (raw)
In-Reply-To: <20240517120234.GA32598@willie-the-truck>

On 17/05/2024 1:02 pm, Will Deacon wrote:
> On Tue, May 14, 2024 at 11:00:50AM -0700, Namhyung Kim wrote:
>> for_each_sibling_event() checks leader's ctx but it doesn't have the ctx
>> yet if it's the leader.  Like in perf_event_validate_size(), we should
>> skip checking siblings in that case.
>>
>> Fixes: f3c0eba287049 ("perf: Add a few assertions")
>> Reported-by: Greg Thelen <gthelen@google.com>
>> Cc: Robin Murphy <robin.murphy@arm.com>
>> Cc: Tuan Phan <tuanphan@os.amperecomputing.com>
>> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
>> ---
>>   drivers/perf/arm_dmc620_pmu.c | 9 ++++++---
>>   1 file changed, 6 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/perf/arm_dmc620_pmu.c b/drivers/perf/arm_dmc620_pmu.c
>> index 8a81be2dd5ec..88c17c1d6d49 100644
>> --- a/drivers/perf/arm_dmc620_pmu.c
>> +++ b/drivers/perf/arm_dmc620_pmu.c
>> @@ -542,12 +542,16 @@ static int dmc620_pmu_event_init(struct perf_event *event)
>>   	if (event->cpu < 0)
>>   		return -EINVAL;
>>   
>> +	hwc->idx = -1;
>> +
>> +	if (event->group_leader == event)
>> +		return 0;
>> +
>>   	/*
>>   	 * We can't atomically disable all HW counters so only one event allowed,
>>   	 * although software events are acceptable.
>>   	 */
>> -	if (event->group_leader != event &&
>> -			!is_software_event(event->group_leader))
>> +	if (!is_software_event(event->group_leader))
>>   		return -EINVAL;

Oh, come to think of it, I believe we shouldn't actually need to keep 
this check either, since commit bf480f938566 ("perf/core: Don't allow 
grouping events from different hw pmus").

Thanks,
Robin.

>>   
>>   	for_each_sibling_event(sibling, event->group_leader) {
>> @@ -556,7 +560,6 @@ static int dmc620_pmu_event_init(struct perf_event *event)
>>   			return -EINVAL;
>>   	}
>>   
>> -	hwc->idx = -1;
>>   	return 0;
>>   }
> 
> Thanks, I'll pick this up, although Mark reckoned he'd found some other
> issues over at:
> 
> https://lore.kernel.org/r/Zg0l642PgQ7T3a8Z@FVFF77S0Q05N
> 
> but didn't elaborate on what exactly he'd found :/
> 
> Will
> 
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> linux-arm-kernel@lists.infradead.org
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