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

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

On Tue, 30 Jan 2024 09:34:29 +0100
Daniel Vetter <daniel@ffwll.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@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.  
> > 
> > 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.

> 
> 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 :-/

Totally agree with you on that point.