[PATCH 2/4] drm/amdgpu: Optimization of critical section

[axie]

On 2017-06-12 06:47 PM, Christian König wrote:
> Am 12.06.2017 um 22:31 schrieb Alex Xie:
>> Make the critical section smaller. There is no
>> need to protect the bo_list, because there is
>> no other task can access the newly created BO
>> list yet.
>
> NAK, a task can guess the next id number so could get the kernel to 
> use the structure before it is initialized.
>
> Christian.
>
How did you find such an extreme corner case? I am fine with this 
comment. Tuesday/Wednesday I will address it with next version of patch set.

Currently, there are 2 options:
Option 1: I may use a write_lock in the create function. And restore the 
original code for the creation of BO list.
Option 2: I may move the function call of idr_alloc to the end of the 
creation of BO list ioctl. This is more efficient but
the code look dirty.


>>
>> Signed-off-by: Alex Xie <AlexBin.Xie at amd.com>
>> ---
>>   drivers/gpu/drm/amd/amdgpu/amdgpu_bo_list.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_bo_list.c 
>> b/drivers/gpu/drm/amd/amdgpu/amdgpu_bo_list.c
>> index 02c138f..c994a04 100644
>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_bo_list.c
>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_bo_list.c
>> @@ -48,8 +48,9 @@ static int amdgpu_bo_list_create(struct 
>> amdgpu_fpriv *fpriv,
>>       mutex_lock(&fpriv->bo_list_lock);
>>       r = idr_alloc(&fpriv->bo_list_handles, *result,
>>                 1, 0, GFP_KERNEL);
>> +    mutex_unlock(&fpriv->bo_list_lock);
>> +
>>       if (r < 0) {
>> -        mutex_unlock(&fpriv->bo_list_lock);
>>           kfree(*result);
>>           return r;
>>       }
>> @@ -60,7 +61,6 @@ static int amdgpu_bo_list_create(struct 
>> amdgpu_fpriv *fpriv,
>>       (*result)->array = NULL;
>>         mutex_lock(&(*result)->lock);
>> -    mutex_unlock(&fpriv->bo_list_lock);
>>         return 0;
>>   }
>
>