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 10:39, Andrzej Hajda wrote:
> On 10.05.2022 10:18, Tvrtko Ursulin wrote:
>>
>> 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?
> 
> intel_gt_tile_cleanup ?

Called from intel_gt_release_all, whose only caller is the failure path 
of i915_driver_probe. Feels like something is missing?

Regards,

Tvrtko