On Thu, Aug 22, 2019 at 10:23:29AM +0200, Christian König wrote: > Am 21.08.19 um 18:04 schrieb Daniel Vetter: > > On Wed, Aug 21, 2019 at 02:31:44PM +0200, Christian König wrote: > > > [SNIP] > > > + /* Try to drop the last reference */ > > > + if (!dma_fence_array_recycle(staged)) > > Without an rcu barrier here you're not syncing to new clients at all. > > I don't think this works, and I expect that once you've readded all the > > barriers and retry loops we're back to seqlocks. > > The key difference is that RCU users now use dma_fence_get_rcu_safe() to > grab a reference to the current set of fences. > > In other words the whole array is reference counted and RCU protected > instead of each individual entry in the array. > > This way you don't need the sequence count any more because you grab a > reference to all of them at once and then can be sure that they don't > change. Hm yeah ... I think there's still some users left that have an open-coded rcu section though. But yeah if you can concince Chris that this is ok I think it makes sense as an overall cleanup of the hand-rolled fences array we have for shared fences. But I'd really like to untangle it from the entire semantics discussion, since that seems entirely unrelated. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
