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

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Date: Fri, 26 Jan 2024 19:43:29 +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 1/26/24 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.

-- 
Best regards,
Dmitry