Am 22.08.19 um 15:02 schrieb Daniel Vetter: > 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. Yeah, agree. To untangle that is a really good idea. Going to send out the dma_fence_array as a replacement for shared fences separately first. Christian. > -Daniel
