Re: [Intel-gfx] [PATCH 6/8] drm/i915/gt: Fix memory leaks in per-gt sysfs

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

On 10/05/2022 08:58, Andrzej Hajda wrote:
> Hi Tvrtko,
> 
> On 10.05.2022 09:28, Tvrtko Ursulin wrote:
>>
>> On 29/04/2022 20:56, Ashutosh Dixit wrote:
>>> All kmalloc'd kobjects need a kobject_put() to free memory. For 
>>> example in
>>> previous code, kobj_gt_release() never gets called. The requirement of
>>> kobject_put() now results in a slightly different code organization.
>>>
>>> v2: s/gtn/gt/ (Andi)
>>>
>>> Cc: Andi Shyti <andi.shyti@intel.com>
>>> Cc: Andrzej Hajda <andrzej.hajda@intel.com>
>>> Fixes: b770bcfae9ad ("drm/i915/gt: create per-tile sysfs interface")
>>> Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
>>> ---
>>>   drivers/gpu/drm/i915/gt/intel_gt.c       |  1 +
>>>   drivers/gpu/drm/i915/gt/intel_gt_sysfs.c | 29 ++++++++++--------------
>>>   drivers/gpu/drm/i915/gt/intel_gt_sysfs.h |  6 +----
>>>   drivers/gpu/drm/i915/gt/intel_gt_types.h |  3 +++
>>>   drivers/gpu/drm/i915/i915_sysfs.c        |  2 ++
>>>   5 files changed, 19 insertions(+), 22 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/i915/gt/intel_gt.c 
>>> b/drivers/gpu/drm/i915/gt/intel_gt.c
>>> index 92394f13b42f..9aede288eb86 100644
>>> --- a/drivers/gpu/drm/i915/gt/intel_gt.c
>>> +++ b/drivers/gpu/drm/i915/gt/intel_gt.c
>>> @@ -785,6 +785,7 @@ void intel_gt_driver_unregister(struct intel_gt *gt)
>>>   {
>>>       intel_wakeref_t wakeref;
>>>   +    intel_gt_sysfs_unregister(gt);
>>>       intel_rps_driver_unregister(&gt->rps);
>>>       intel_gsc_fini(&gt->gsc);
>>>   diff --git a/drivers/gpu/drm/i915/gt/intel_gt_sysfs.c 
>>> b/drivers/gpu/drm/i915/gt/intel_gt_sysfs.c
>>> index 8ec8bc660c8c..9e4ebf53379b 100644
>>> --- a/drivers/gpu/drm/i915/gt/intel_gt_sysfs.c
>>> +++ b/drivers/gpu/drm/i915/gt/intel_gt_sysfs.c
>>> @@ -24,7 +24,7 @@ bool is_object_gt(struct kobject *kobj)
>>>     static struct intel_gt *kobj_to_gt(struct kobject *kobj)
>>>   {
>>> -    return container_of(kobj, struct kobj_gt, base)->gt;
>>> +    return container_of(kobj, struct intel_gt, sysfs_gt);
>>>   }
>>>     struct intel_gt *intel_gt_sysfs_get_drvdata(struct device *dev,
>>> @@ -72,9 +72,9 @@ static struct attribute *id_attrs[] = {
>>>   };
>>>   ATTRIBUTE_GROUPS(id);
>>>   +/* A kobject needs a release() method even if it does nothing */
>>>   static void kobj_gt_release(struct kobject *kobj)
>>>   {
>>> -    kfree(kobj);
>>>   }
>>>     static struct kobj_type kobj_gt_type = {
>>> @@ -85,8 +85,6 @@ static struct kobj_type kobj_gt_type = {
>>>     void intel_gt_sysfs_register(struct intel_gt *gt)
>>>   {
>>> -    struct kobj_gt *kg;
>>> -
>>>       /*
>>>        * We need to make things right with the
>>>        * ABI compatibility. The files were originally
>>> @@ -98,25 +96,22 @@ void intel_gt_sysfs_register(struct intel_gt *gt)
>>>       if (gt_is_root(gt))
>>>           intel_gt_sysfs_pm_init(gt, gt_get_parent_obj(gt));
>>>   -    kg = kzalloc(sizeof(*kg), GFP_KERNEL);
>>> -    if (!kg)
>>> +    /* init and xfer ownership to sysfs tree */
>>> +    if (kobject_init_and_add(&gt->sysfs_gt, &kobj_gt_type,
>>> +                 gt->i915->sysfs_gt, "gt%d", gt->info.id))
>>
>> Was there closure/agreement on the matter of whether or not there is a 
>> potential race between "kfree(gt)" and sysfs access (last put from 
>> sysfs that is)? I've noticed Andrzej and Ashutosh were discussing it 
>> but did not read all the details.
>>
> 
> Not really :)
> IMO docs are against this practice, Ashutosh shows examples of this 
> practice in code and according to his analysis it is safe.
> I gave up looking for contradictions :) Either it is OK, kobject is not 
> fully shared object, docs are obsolete and needs update, either the 
> patch is wrong.
> Anyway finally I tend to accept this solution, I failed to prove it is 
> wrong :)

Like a question of whether hotunplug can be triggered while userspace is 
sitting in a sysfs hook? Final kfree then has to be delayed until 
userspace exists.

Btw where is the "kfree(gt)" for the tiles on the PCI remove path? I 
can't find it.. Do we have a leak?

Regards,

Tvrtko