On Thu, Aug 04, 2016 at 11:26:04AM +0300, Joonas Lahtinen wrote: > On ma, 2016-08-01 at 19:22 +0100, 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, and to do this we employ RCU on > > the request cache and upon the last_request pointer tracking. > > > > 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. > > > > The commit message seems little bit disconnect with the code, making > the patch sound much more complex than it is. Is it up to date? Or > maybe parts of this explanation would belong to an earlier patch? Well, you caught me. This used to be quite ugly until it was hidden await behind a new helper, and this code has been through multiple phases gradually removing the struct_mutex further and further. With a couple of glaring exceptions (the bits not talking about the innards but about the top level implementation), it is describing what is going on under the covers. -Chris -- Chris Wilson, Intel Open Source Technology Centre _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/intel-gfx
