Re: [PATCH] drm/i915: Check caller held wakerefs in assert_forcewakes_active

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 03/07/2019 13:48, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2019-07-03 13:43:34)
>>
>> On 03/07/2019 13:12, Chris Wilson wrote:
>>> The intent of the assert is to document that the caller took the
>>> appropriate wakerefs for the function. However, as Tvrtko pointed out,
>>> we simply check whether the fw_domains are active and may be confused by
>>> the auto wakeref which may be dropped between the check and use. Let's
>>> be more careful in the assert and check that each fw_domain has an
>>> explicit caller wakeref above and beyond the automatic wakeref.
>>
>> Although we still don't know if it is our caller who took the fw or some
>> unrelated thread. Still, a more thorough check is better even if not
>> perfect.
> 
> Since it's not a mutex, we can't stuff an owner field in here, the only
> way to properly track in that case would be to return cookies from
> forcewake_get and verify the cookies are still active.
> 
> Not feeling the inclination to do that yet. Maybe if we get a few fw
> leaks.

Agreed, I wasn't trying to say we have to do it.

> 
>>> Reported-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>>> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>> Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
>>> ---
>>>    drivers/gpu/drm/i915/intel_uncore.c | 24 ++++++++++++++++++++++++
>>>    1 file changed, 24 insertions(+)
>>>
>>> diff --git a/drivers/gpu/drm/i915/intel_uncore.c b/drivers/gpu/drm/i915/intel_uncore.c
>>> index 68d54e126d79..bc25a6e51463 100644
>>> --- a/drivers/gpu/drm/i915/intel_uncore.c
>>> +++ b/drivers/gpu/drm/i915/intel_uncore.c
>>> @@ -738,6 +738,12 @@ void assert_forcewakes_inactive(struct intel_uncore *uncore)
>>>    void assert_forcewakes_active(struct intel_uncore *uncore,
>>>                              enum forcewake_domains fw_domains)
>>>    {
>>> +     struct intel_uncore_forcewake_domain *domain;
>>> +     unsigned int tmp;
>>> +
>>> +     if (!IS_ENABLED(CONFIG_DRM_i915_RUNTIME_PM))
>>> +             return;
>>> +
>>
>> If uncore->funcs.force_wake_get is set why wouldn't we still want to run
>> the asserts?
> 
> I'm just being worried by adding a loop under irq-off and didn't want to
> add more trouble to non-debug kernels. (Closing the stable door much?)

What is the connection between debug/non-debug kernels and 
CONFIG_DRM_i915_RUNTIME_PM?

Regards,

Tvrtko

>>
>>>        if (!uncore->funcs.force_wake_get)
>>>                return;
>>>    
>>> @@ -747,6 +753,24 @@ void assert_forcewakes_active(struct intel_uncore *uncore,
>>>        WARN(fw_domains & ~uncore->fw_domains_active,
>>>             "Expected %08x fw_domains to be active, but %08x are off\n",
>>>             fw_domains, fw_domains & ~uncore->fw_domains_active);
>>> +
>>> +     /*
>>> +      * Check that the caller has an explicit wakeref and we don't mistake
>>> +      * it for the auto wakeref.
>>> +      */
>>> +     local_irq_disable();
>>> +     for_each_fw_domain_masked(domain, fw_domains, uncore, tmp) {
>>> +             unsigned int expect = 1;
>>> +
>>> +             if (hrtimer_active(&domain->timer))
>>> +                     expect++;
>>> +
>>> +             if (WARN(domain->wake_count < expect,
>>> +                      "Expected domain %d to be held awake by caller\n",
>>> +                      domain->id))
>>> +                     break;
>>> +     }
>>> +     local_irq_enable();
>>
>> This part looks good. Let wait and see if CI calls me a fool.
> 
> Aye, that's what I'm waiting for as well. Personalized insults from CI :)
> -Chris
> 
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