[PATCH 2/9] drm/syncobj: Lock around drm_syncobj::fence

[Chris Wilson]

Quoting Jason Ekstrand (2017-08-10 01:31:52)
> 
> 
> On Wed, Aug 9, 2017 at 2:21 PM, Chris Wilson <chris at chris-wilson.co.uk> wrote:
> 
>     Quoting Jason Ekstrand (2017-08-08 23:46:02)
>     > The atomic exchange operation we were doing before in replace_fence was
>     > sufficient for the case where it raced with itself.  However, if you
>     > have a race between a replace_fence and dma_fence_get(syncobj->fence),
>     > you may end up with the entire replace_fence happening between the point
>     > in time where the one thread gets the syncobj->fence pointer and when it
>     > calls dma_fence_get() on it.  If this happens, then the reference may be
>     > dropped before we get a chance to get a new one.
> 
>     This doesn't require a spinlock, just dma_fence_get_rcu_safe(). The
>     argument for keeping this patch lies in the merit of later patches..
> 
> 
> Doesn't that also require that we start using an RCU for syncobj?  That was my
> interpretation of the hieroglyphics above the definition of get_rcu_safe()

That we use RCU for the fence, which we do. i.e. so that the fence
pointer remains valid during the atomic_inc_unless_zero. The caller is
responsible for making sure that the syncobj remains valid across the
call (which we do using a ref, but it too could use RCU if that was ever
desired).
-Chris