On Wed, Dec 23, 2015 at 11:19:23AM +0000, Chris Wilson wrote: > With a bit of care (and leniency) we can iterate over the object and > wait for previous rendering to complete with judicial use of atomic > reference counting. The ABI requires us to ensure that an active object > is eventually flushed (like the busy-ioctl) which is guaranteed by our > management of requests (i.e. everything that is submitted to hardware is > flushed in the same request). All we have to do is ensure that we can > detect when the requests are complete for reporting when the object is > idle (without triggering ETIME) - this is handled by > __i915_wait_request. > > The biggest danger in the code is walking the object without holding any > locks. We iterate over the set of last requests and carefully grab a > reference upon it. (If it is changing beneath us, that is the usual > userspace race and even with locking you get the same indeterminate > results.) If the request is unreferenced beneath us, it will be disposed > of into the request cache - so we have to carefully order the retrieval > of the request pointer with its removal. > > The impact of this is actually quite small - the return to userspace > following the wait was already lockless. What we achieve here is > completing an already finished wait without hitting the struct_mutex, > our hold is quite short and so we are typically just a victim of > contention rather than a cause. > > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> Some food for thought. I would especially like someone to poke holes in the racy pointer lookup and check the store mb() versus the rcu-reference. The same technique would also apply to busy-ioctl, and that's about the limits of its applicablity. Though isolating those from contention and having to do a flush-active should be beneficial system-wide. -Chris -- Chris Wilson, Intel Open Source Technology Centre _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/intel-gfx
