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

[Janusz Krzysztofik]

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?

Thanks,
Janusz

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