linux-kernel - Re: [PATCH v19 09/30] drm/shmem-helper: Add and use lockless drm_gem_shmem_get

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Date: Thu, 1 Feb 2024 21:53:02 +0300
From: Dmitry Osipenko <dmitry.osipenko@...labora.com>
To: Boris Brezillon <boris.brezillon@...labora.com>,
 Daniel Vetter <daniel@...ll.ch>
Cc: David Airlie <airlied@...il.com>, Gerd Hoffmann <kraxel@...hat.com>,
 Gurchetan Singh <gurchetansingh@...omium.org>, Chia-I Wu
 <olvaffe@...il.com>, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 Christian König <christian.koenig@....com>,
 Qiang Yu <yuq825@...il.com>, Steven Price <steven.price@....com>,
 Emma Anholt <emma@...olt.net>, Melissa Wen <mwen@...lia.com>,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 kernel@...labora.com, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v19 09/30] drm/shmem-helper: Add and use lockless
 drm_gem_shmem_get_pages()

On 1/30/24 13:10, Boris Brezillon wrote:
> On Tue, 30 Jan 2024 09:34:29 +0100
> Daniel Vetter <daniel@...ll.ch> 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@...ll.ch> 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@...nel.org>
>>>>>> Reviewed-by: Boris Brezillon <boris.brezillon@...labora.com>
>>>>>> Suggested-by: Boris Brezillon <boris.brezillon@...labora.com>
>>>>>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@...labora.com>
>>>>>> ---
>>>>>>  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.
> 
> Yeah, I also considered adding a dedicated function for that use case
> at some point, instead of abusing get_pages(). Given I no longer need
> it, we can probably add this might_lock() and defer the addition of this
> get_borrowed_pages() helper until someone actually needs it.

Ack, I'll add the might_lock() then. Missed previously that you don't
need to use get_pages() anymore. Thanks

-- 
Best regards,
Dmitry