[Mesa-dev] [PATCH v2 07/18] anv/allocator: Add a BO cache

[Chris Wilson]

On Wed, Mar 15, 2017 at 09:50:57AM +0000, Chris Wilson wrote:
> On Tue, Mar 14, 2017 at 10:43:07PM -0700, Jason Ekstrand wrote:
> > +void
> > +anv_bo_cache_release(struct anv_device *device,
> > +                     struct anv_bo_cache *cache,
> > +                     struct anv_bo *bo_in,
> > +                     VkAllocationCallbacks *alloc)
> > +{
> > +   assert(anv_bo_cache_lookup(cache, bo_in->gem_handle) == bo_in);
> > +   struct anv_cached_bo *bo = (struct anv_cached_bo *)bo_in;
> > +
> > +   uint32_t count = __sync_fetch_and_add(&bo->refcount, -1);
> > +   assert(count > 0);
> > +   if (count > 1)
> > +      return;
> > +   assert(count == 1);
> > +   assert(bo->refcount == 0);
> 
> There's a window here for a second thread to acquire a reference to the
> bo, just before you then go and free it.
> 
> The trick is the final unref has to be inside the mutex, earlier can be
> outside. See refcount_dec_and_lock() in the kernel.

The alternative is to check against the dead bo when retrieving from the
cache (under the mutex) -- refcount_add_not_zero. That is probably the
cheaper option.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre