Re: [PATCH v19 09/30] drm/shmem-helper: Add and use lockless drm_gem_shmem_get_pages()

[Dmitry Osipenko]

On 1/30/24 11:34, Daniel Vetter wrote:
> On Fri, Jan 26, 2024 at 07:43:29PM +0300, Dmitry Osipenko wrote:
>> On 1/26/24 13:18, Boris Brezillon wrote:
>>> On Thu, 25 Jan 2024 18:24:04 +0100
>>> Daniel Vetter <daniel@xxxxxxxx> wrote:
>>>
>>>> On Fri, Jan 05, 2024 at 09:46:03PM +0300, Dmitry Osipenko wrote:
>>>>> Add lockless drm_gem_shmem_get_pages() helper that skips taking reservation
>>>>> lock if pages_use_count is non-zero, leveraging from atomicity of the
>>>>> refcount_t. Make drm_gem_shmem_mmap() to utilize the new helper.
>>>>>
>>>>> Acked-by: Maxime Ripard <mripard@xxxxxxxxxx>
>>>>> Reviewed-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
>>>>> Suggested-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
>>>>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx>
>>>>> ---
>>>>>  drivers/gpu/drm/drm_gem_shmem_helper.c | 19 +++++++++++++++----
>>>>>  1 file changed, 15 insertions(+), 4 deletions(-)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
>>>>> index cacf0f8c42e2..1c032513abf1 100644
>>>>> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
>>>>> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
>>>>> @@ -226,6 +226,20 @@ void drm_gem_shmem_put_pages_locked(struct drm_gem_shmem_object *shmem)
>>>>>  }
>>>>>  EXPORT_SYMBOL_GPL(drm_gem_shmem_put_pages_locked);
>>>>>  
>>>>> +static int drm_gem_shmem_get_pages(struct drm_gem_shmem_object *shmem)
>>>>> +{
>>>>> +	int ret;  
>>>>
>>>> Just random drive-by comment: a might_lock annotation here might be good,
>>>> or people could hit some really interesting bugs that are rather hard to
>>>> reproduce ...
>>>
>>> Actually, being able to acquire a ref in a dma-signalling path on an
>>> object we know for sure already has refcount >= 1 (because we previously
>>> acquired a ref in a path where dma_resv_lock() was allowed), was the
>>> primary reason I suggested moving to this atomic-refcount approach.
>>>
>>> In the meantime, drm_gpuvm has evolved in a way that allows me to not
>>> take the ref in the dma-signalling path (the gpuvm_bo object now holds
>>> the ref, and it's acquired/released outside the dma-signalling path).
>>>
>>> Not saying we shouldn't add this might_lock(), but others might have
>>> good reasons to have this function called in a path where locking
>>> is not allowed.
>>
>> For Panthor the might_lock indeed won't be a appropriate, thanks for
>> reminding about it. I'll add explanatory comment to the code.
> 
> Hm these kind of tricks feel very dangerous to me. I think it would be
> good to split up the two cases into two functions:
> 
> 1. first one does only the atomic_inc and splats if the refcount is zero.
> I think something in the name that denotes that we're incrementing a
> borrowed pages reference would be good here, so like get_borrowed_pages
> (there's not really a naming convention for these in the kernel).
> Unfortunately no rust so we can't enforce that you provide the right kind
> of borrowed reference at compile time.
> 
> 2. second one has the might_lock.
> 
> This way you force callers to think what they're doing and ideally
> document where the borrowed reference is from, and ideally document that
> in the code. Otherwise we'll end up with way too much "works in testing,
> but is a nice CVE" code :-/

We indeed can have both variants of the borrowed/non-borrowed functions.
Thanks again for the suggestions

-- 
Best regards,
Dmitry