Quoting Tvrtko Ursulin (2019-10-04 10:15:20)
>
> On 03/10/2019 22:01, Chris Wilson wrote:
> > A few callers need to serialise the destruction of their drm_mm_node and
> > ensure it is removed from the drm_mm before freeing. However, to be
> > completely sure that any access from another thread is complete before
> > we free the struct, we require the RELEASE semantics of
> > clear_bit_unlock().
> >
> > This allows the conditional locking such as
> >
> > Thread A Thread B
> > mutex_lock(mm_lock); if (drm_mm_node_allocated(node)) {
> > drm_mm_node_remove(node); mutex_lock(mm_lock);
> > mutex_unlock(mm_lock); drm_mm_node_remove(node);
> > mutex_unlock(mm_lock);
> > }
> > kfree(node);
>
> My understanding is that release semantics on node allocated mean 1 -> 0
> transition is guaranteed to be seen only when thread A
> drm_mm_node_remove is fully done with the removal. But if it is in the
> middle of removal, node is still seen as allocated outside and thread B
> can enter the if-body, wait for the lock, and then drm_mm_node_remove
> will attempt a double removal. So I think another drm_mm_node_allocated
> under the lock is needed.
Yes. Check after the lock is indeed required in this scenario. And
drm_mm_node_remove() insists the caller doesn't try a double remove.
-Chris
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