Hey,
Op 11-08-12 21:39, Daniel Vetter schreef:
if (!ret) {cb->base.flags = 0;cb->base.func = __dma_fence_wake_func;cb->base.private = priv;cb->fence = fence;cb->func = func;__add_wait_queue(&fence->event_queue, &cb->base);}spin_unlock_irqrestore(&fence->event_queue.lock, flags);return ret;+} +EXPORT_SYMBOL_GPL(dma_fence_add_callback);
I think for api completenes we should also have a dma_fence_remove_callback function.
We did originally but Maarten found it was difficult to deal with properly when the gpu's hang. I think his alternative was just to require the hung driver to signal the fence. I had kicked around the idea of a dma_fence_cancel() alternative to signal that could pass an error thru to the waiting driver.. although not sure if the other driver could really do anything differently at that point.
No, there is a very real reason I removed dma_fence_remove_callback. It is absolutely non-trivial to cancel it once added, since you have to deal with all kinds of race conditions.. See i915_gem_reset_requests in my git tree: http://cgit.freedesktop.org/~mlankhorst/linux/commit/?id=673c4b2550bc63ec134...
I don't see the point in that code ... Why can't we drop the kref _outside_ of the critical section protected by event_queue_lock? Then you pretty much have an open-coded version of dma_fence_callback_cancel in there.
The event_queue_lock protects 2 things: 1. Refcount to dma_fence won't drop to 0 if val->fences[i] != NULL Creator is supposed to keep a refcount until after dma_fence_signal returns. Adding a refcount you release in the callback won't help you here much.
2. Integrity of request->prime_list The list_del's are otherwise not serialized, leaving a corrupted list if 2 fences signal at the same time. kref_put in the non-free'ing case is simply an atomic decrement, so there's no measurable penalty for keeping it in the lock.
So no, you could remove it from the kref_put, but val->fences[i] = NULL assignment would still need it, so there's no real penalty left for putting kref_put in the spinlock to also protect the second case without dropping/retaking lock.
I'll add dma_fence_remove_callback that returns a bool of whether the callback was removed or not. In the latter case the fence already fired. However, if you call dma_fence_remove_callback twice, on the wrong fence, or without ever calling dma_fence_add_callback you'd undefined behavior, and there's no guarantee I could detech such situation, but with those constraints I think it could be useful to have.
It sucks but prime_rm_lock is the inner lock so the only way not to deadlock is doing what I'm doing there, or not getting the hardware locked in the first place.
This is the only way to do it completely deadlock/memory corruption free since you essentially have a locking inversion to avoid. I had it wrong the first 2 times too, even when I knew about a lot of the locking complications. If you want to do it, in most cases it will likely be easier to just eat the signal and ignore it instead of canceling.
+/**
- dma_fence_wait - wait for a fence to be signaled
- @fence: [in] The fence to wait on
- @intr: [in] if true, do an interruptible wait
- @timeout: [in] absolute time for timeout, in jiffies.
I don't quite like this, I think we should keep the styl of all other wait_*_timeout functions and pass the arg as timeout in jiffies (and also the same return semantics). Otherwise well have funny code that needs to handle return values differently depending upon whether it waits upon a dma_fence or a native object (where it would us the wait_*_timeout functions directly).
We did start out this way, but there was an ugly jiffies roll-over problem that was difficult to deal with properly. Using an absolute time avoided the problem.
Yeah, this makes it easier to wait on multiple fences, instead of resetting the timeout over and over and over again, or manually recalculating.
I don't see how updating the jiffies_left timeout is that onerous, and in any case we can easily wrap that up into a little helper function, passing in an array of dma_fence pointers.
Creating interfaces that differ from established kernel api patterns otoh isn't good imo. I.e. I want dma_fence_wait_bla to be a drop-in replacement for the corresponding wait_event_bla function/macro, which the same semantics for the timeout and return values.
Differing in such things only leads to confusion when reading patches imo.
Ok, I'll see if I can make a set of functions that follow the normal rules for these types of functions.
~Maarten