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

[Boris Brezillon <boris.brezillon@collabora.com>]

On Thu, 25 Jan 2024 18:24:04 +0100
Daniel Vetter <daniel@ffwll.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@kernel.org>
> > Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
> > Suggested-by: Boris Brezillon <boris.brezillon@collabora.com>
> > Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.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.