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

[Janusz Krzysztofik]

On Friday, 10 March 2023 17:48:10 CET Das, Nirmoy wrote:
> 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.

Looking at 5.10, I can't understand how dropping the check instead of 
replacing it with a still problematic one could make backporting less easy.

Thanks,
Janusz


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