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

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

[Namhyung Kim]

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;
 
 	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;
 }
 
-- 
2.45.0.rc1.225.g2a3ae87e7f-goog

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

[Robin Murphy]

On 2024-05-14 7:00 pm, 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.

Ugh, looking around for_each_sibling_event() sites, it looks like there 
are a fair few other drivers using this pattern as well :(

I'd love for groups to be less horribly complicated, but I think I can 
follow the underlying reasoning here. I suppose one could argue that the 
assertion could take into account that there's nothing to protect in the 
case where event->ctx is still NULL, since nobody else should be able to 
touch the event's own empty sibling list at this point before 
perf_event_open() has even returned. However by the same token there's 
also no real reason for drivers *not* to return early when they equally 
can tell that the sibling list must be empty, and indeed that seems to 
be a fairly common pattern too, so I see no issue with fixing up all the 
offending drivers for consistency either.

Reviewed-by: Robin Murphy <robin.murphy@arm.com>

> 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;
>   
>   	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;
>   }
>   

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

[Will Deacon]

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;
>  
>  	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

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

[Robin Murphy]

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
> 
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

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

[Mark Rutland]

On Fri, May 17, 2024 at 01:02:34PM +0100, 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;
> >  
> >  	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 :/

Sorry; what I was referring to was that some drivers (including this
one) also forgot to validate that the group size could actually fit on
the PMU, which we're *supposed* to check, but if we fail to do so the
only fallout is that events won't count and we waste a bit of time
trying to schedule events unnecessarily.

For this patch as-is:

Acked-by: Mark Rutland <mark.rutland@arm.com>

... and I'll try to take a look at the rest when I'm back in the UK.

Mark.

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

[Will Deacon]

On Tue, 14 May 2024 11:00:50 -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.
> 
> 

Applied to arm64 (for-next/core), thanks!

[1/1] perf/arm-dmc620: Fix lockdep assert in ->event_init()
      https://git.kernel.org/arm64/c/a4c5a457c610

Cheers,
-- 
Will

https://fixes.arm64.dev
https://next.arm64.dev
https://will.arm64.dev