| Message ID | 20250409120640.106408-2-phasta@kernel.org (mailing list archive) |
|---|---|
| Headers | show |
| Series | dma-fence: Rename dma_fence_is_signaled() | expand |
On Wed, 2025-04-09 at 14:51 +0200, Philipp Stanner wrote: > On Wed, 2025-04-09 at 14:39 +0200, Boris Brezillon wrote: > > Hi Philipp, > > > > On Wed, 9 Apr 2025 14:06:37 +0200 > > Philipp Stanner <phasta@kernel.org> wrote: > > > > > dma_fence_is_signaled()'s name strongly reads as if this function > > > were > > > intended for checking whether a fence is already signaled. Also > > > the > > > boolean it returns hints at that. > > > > > > The function's behavior, however, is more complex: it can check > > > with a > > > driver callback whether the hardware's sequence number indicates > > > that > > > the fence can already be treated as signaled, although the > > > hardware's / > > > driver's interrupt handler has not signaled it yet. If that's the > > > case, > > > the function also signals the fence. > > > > > > (Presumably) this has caused a bug in Nouveau (unknown commit), > > > where > > > nouveau_fence_done() uses the function to check a fence, which > > > causes a > > > race. > > > > > > Give the function a more obvious name. > > > > This is just my personal view on this, but I find the new name just > > as > > confusing as the old one. It sounds like something is checked, but > > it's > > clear what, and then the fence is forcibly signaled like it would > > be > > if > > you call drm_fence_signal(). Of course, this clarified by the doc, > > but > > given the goal was to make the function name clearly reflect what > > it > > does, I'm not convinced it's significantly better. > > > > Maybe dma_fence_check_hw_state_and_propagate(), though it might be > > too long of name. Oh well, feel free to ignore this comments if a > > majority is fine with the new name. > > Yoa, the name isn't perfect (the perfect name describing the whole > behavior would be > dma_fence_check_if_already_signaled_then_check_hardware_state_and_pro > pa > gate() ^^' > > My intention here is to have the reader realize "watch out, the fence > might get signaled here!", which is probably the most important event > regarding fences, which can race, invoke the callbacks and so on. > > For details readers will then check the documentation. > > But I'm of course open to see if there's a majority for this or that > name. how about: dma_fence_check_hw_and_signal() ? P. > > P. > > > > > > Regards, > > > > Boris >
On Wed, 2025-04-09 at 16:10 +0200, Christian König wrote: > Am 09.04.25 um 16:01 schrieb Philipp Stanner: > > On Wed, 2025-04-09 at 15:14 +0200, Christian König wrote: > > > Am 09.04.25 um 14:56 schrieb Philipp Stanner: > > > > On Wed, 2025-04-09 at 14:51 +0200, Philipp Stanner wrote: > > > > > On Wed, 2025-04-09 at 14:39 +0200, Boris Brezillon wrote: > > > > > > Hi Philipp, > > > > > > > > > > > > On Wed, 9 Apr 2025 14:06:37 +0200 > > > > > > Philipp Stanner <phasta@kernel.org> wrote: > > > > > > > > > > > > > dma_fence_is_signaled()'s name strongly reads as if this > > > > > > > function > > > > > > > were > > > > > > > intended for checking whether a fence is already > > > > > > > signaled. > > > > > > > Also > > > > > > > the > > > > > > > boolean it returns hints at that. > > > > > > > > > > > > > > The function's behavior, however, is more complex: it can > > > > > > > check > > > > > > > with a > > > > > > > driver callback whether the hardware's sequence number > > > > > > > indicates > > > > > > > that > > > > > > > the fence can already be treated as signaled, although > > > > > > > the > > > > > > > hardware's / > > > > > > > driver's interrupt handler has not signaled it yet. If > > > > > > > that's > > > > > > > the > > > > > > > case, > > > > > > > the function also signals the fence. > > > > > > > > > > > > > > (Presumably) this has caused a bug in Nouveau (unknown > > > > > > > commit), > > > > > > > where > > > > > > > nouveau_fence_done() uses the function to check a fence, > > > > > > > which > > > > > > > causes a > > > > > > > race. > > > > > > > > > > > > > > Give the function a more obvious name. > > > > > > This is just my personal view on this, but I find the new > > > > > > name > > > > > > just > > > > > > as > > > > > > confusing as the old one. It sounds like something is > > > > > > checked, > > > > > > but > > > > > > it's > > > > > > clear what, and then the fence is forcibly signaled like it > > > > > > would > > > > > > be > > > > > > if > > > > > > you call drm_fence_signal(). Of course, this clarified by > > > > > > the > > > > > > doc, > > > > > > but > > > > > > given the goal was to make the function name clearly > > > > > > reflect > > > > > > what > > > > > > it > > > > > > does, I'm not convinced it's significantly better. > > > > > > > > > > > > Maybe dma_fence_check_hw_state_and_propagate(), though it > > > > > > might > > > > > > be > > > > > > too long of name. Oh well, feel free to ignore this > > > > > > comments if > > > > > > a > > > > > > majority is fine with the new name. > > > > > Yoa, the name isn't perfect (the perfect name describing the > > > > > whole > > > > > behavior would be > > > > > dma_fence_check_if_already_signaled_then_check_hardware_state > > > > > _and > > > > > _pro > > > > > pa > > > > > gate() ^^' > > > > > > > > > > My intention here is to have the reader realize "watch out, > > > > > the > > > > > fence > > > > > might get signaled here!", which is probably the most > > > > > important > > > > > event > > > > > regarding fences, which can race, invoke the callbacks and so > > > > > on. > > > > > > > > > > For details readers will then check the documentation. > > > > > > > > > > But I'm of course open to see if there's a majority for this > > > > > or > > > > > that > > > > > name. > > > > how about: > > > > > > > > dma_fence_check_hw_and_signal() ? > > > I don't think that renaming the function is a good idea in the > > > first > > > place. > > > > > > What the function does internally is an implementation detail of > > > the > > > framework. > > > > > > For the code using this function it's completely irrelevant if > > > the > > > function might also signal the fence, what matters for the caller > > > is > > > the returned status of the fence. I think this also counts for > > > the > > > dma_fence_is_signaled() documentation. > > It does obviously matter. As it's currently implemented, a lot of > > important things happen implicitly. > > Yeah, but that's ok. > > The code who calls this is the consumer of the interface and so > shouldn't need to know this. That's why we have created the DMA fence > framework in the first place. > > For the provider side when a driver or similar implements the > interface the relevant documentation is the dma_fence_ops structure. > > > I only see improvement by making things more obvious. > > > > In any case, how would you call a wrapper that just does > > test_bit(IS_SIGNALED, …) ? > > Broken, that was very intentionally removed quite shortly after we > created the framework. > > We have a few cases were implementations do check that for their > fences, but consumers should never be allowed to touch such > internals. There is theory and there is practice. In practice, those internals are being used by Nouveau, i915, Xe, vmgfx and radeon. So it seems that we failed quite a bit at communicating clearly how the interface should be used. And, to repeat myself, with both name and docu of that function, I think it is very easy to misunderstand what it's doing. You say that it shouldn't matter – and maybe that's true, in theory. In practice, it does matter. In practice, APIs get misused and have side-effects. And making that harder is desirable. In any case, I might have to add another such call to Nouveau, because the solution preferred by you over the callback causes another race. Certainly one could solve this in a clean way, but someone has to do the work, and we're talking about more than a few hours here. In any case, be so kind and look at patch 2 and tell me there if you're at least OK with making the documentation more detailed. P. > > Regards, > Christian. > > > > > P. > > > > > What we should improve is the documentation of the dma_fence_ops- > > > > enable_signaling and dma_fence_ops->signaled callbacks. > > > Especially see the comment about reference counts on > > > enable_signaling > > > which is missing on the signaled callback. That is most likely > > > the > > > root cause why nouveau implemented enable_signaling correctly but > > > not > > > the other one. > > > > > > But putting that aside I think we should make nails with heads > > > and > > > let the framework guarantee that the fences stay alive until they > > > are > > > signaled (one way or another). This completely removes the burden > > > to > > > keep a reference on unsignaled fences from the drivers / > > > implementations and make things more over all more defensive. > > > > > > Regards, > > > Christian. > > > > > > > P. > > > > > > > > > P. > > > > > > > > > > > > > > > > Regards, > > > > > > > > > > > > Boris >