[Intel-gfx] [PATCH] drm/i915/active: Fix missing debug object activation

[Das, Nirmoy]

Hi Janusz,

On 3/10/2023 4:19 PM, Janusz Krzysztofik wrote:
> Hi Nirmoy,
>
> On Friday, 10 March 2023 15:11:38 CET Nirmoy Das wrote:
>> debug_active_activate() expected ref->count to be zero
>> which is not true anymore as __i915_active_activate() calls
>> debug_active_activate() after incrementing the count.
>>
>> Fixes: 04240e30ed06 ("drm/i915: Skip taking acquire mutex for no ref->active
> callback")
>> Cc: Chris Wilson <chris at chris-wilson.co.uk>
>> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>> Cc: Thomas Hellström <thomas.hellstrom at intel.com>
>> Cc: Andi Shyti <andi.shyti at linux.intel.com>
>> Cc: intel-gfx at lists.freedesktop.org
>> Cc: <stable at vger.kernel.org> # v5.10+
>> Signed-off-by: Nirmoy Das <nirmoy.das at intel.com>
>> ---
>>   drivers/gpu/drm/i915/i915_active.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/
> i915_active.c
>> index a9fea115f2d2..1c3066eb359a 100644
>> --- a/drivers/gpu/drm/i915/i915_active.c
>> +++ b/drivers/gpu/drm/i915/i915_active.c
>> @@ -92,7 +92,7 @@ static void debug_active_init(struct i915_active *ref)
>>   static void debug_active_activate(struct i915_active *ref)
>>   {
>>   	lockdep_assert_held(&ref->tree_lock);
>> -	if (!atomic_read(&ref->count)) /* before the first inc */
>> +	if (atomic_read(&ref->count) == 1) /* after the first inc */
> While that's obviously better than never calling debug_active_activate(), I'm
> wondering how likely we can still miss it because the counter being
> incremented, e.g. via i915_active_acquire_if_busy(), by a concurrent thread.
> Since __i915_active_activate() is the only user and its decision making step
> is serialized against itself with a spinlock, couldn't we better call
> debug_object_activate() unconditionally here?


Yes, we can call debug_object_activate() without checking ref->count. 
Also we can remove the ref-count check for

debug_active_deactivate() as this is wrapped with 
"atomic_dec_and_lock_irqsave(&ref->count, &ref->tree_lock, flags)".


I think it makes sense to keep this patch as it is so it can be 
backported easily. I can add another patch to remove

unnecessary ref->count  checks.


Regards,

Nirmoy


>
> Thanks,
> Janusz
>
>>   		debug_object_activate(ref, &active_debug_desc);
>>   }
>>   
>>
>
>
>