On Fri, 2025-04-11 at 13:05 +0200, Christian König wrote: > Am 11.04.25 um 11:29 schrieb Philipp Stanner: > > > [SNIP] > > > > It could be, however, that at the same moment > > nouveau_fence_signal() is > > removing that entry, holding the appropriate lock. > > > > So we have a race. Again. > > > > Ah, yes of course. If signaled is called with or without the lock is > actually undetermined. > > > > > > You see, fixing things in Nouveau is difficult :) > > It gets more difficult if you want to clean it up "properly", so it > > conforms to rules such as those from dma_fence. > > > > I have now provided two fixes that both work, but you are not > > satisfied > > with from the dma_fence-maintainer's perspective. I understand > > that, > > but please also understand that it's actually not my primary task > > to > > work on Nouveau. I just have to fix this bug to move on with my > > scheduler work. > > > > Well I'm happy with whatever solution as long as it works, but as > far as I can see the approach with the callback simply doesn't. > > You just can't drop the fence reference for the list from the > callback. > > > > > > So if you have another idea, feel free to share it. But I'd like to > > know how we can go on here. > > > > Well the fence code actually works, doesn't it? The problem is > rather that setting the error throws a warning because it doesn't > expect signaled fences on the pending list. > > Maybe we should fix that instead.
The fence code works as the author intended, but I would be happy if it were more explicitly documented. Regarding the WARN_ON: It occurs in dma_fence_set_error() because there is an attempt to set an error code on a signaled fence. I don't think that should be "fixed", it works as intended: You must not set an error code of a fence that was already signaled. The reason seems to be that once a fence is signaled, a third party might evaluate the error code. But I think this wasn't wat you meant with "fix". In any case, there must not be signaled fences in nouveau's pending- list. They must be removed immediately once they signal, and this must not race. > > > > > > I'm running out of ideas. What I'm wondering if we couldn't just > > remove > > performance hacky fastpath functions such as > > nouveau_fence_is_signaled() completely. It seems redundant to me. > > > > That would work for me as well. I'll test this approach. Seems a bit like the nuclear approach, but if it works we'd at least clean up a lot of this mess. P. > > > > > > > > Or we might add locking to it, but IDK what was achieved with RCU > > here. > > In any case it's definitely bad that Nouveau has so many redundant > > and > > half-redundant mechanisms. > > > > Yeah, agree messing with the locks even more won't help us here. > > Regards, > Christian. > > > > > > > > > > P. > > > > > > > > > > > > > P. > > > > > > > > > > > > > > Regards, > > > > Christian. > > > > > > > > > > > > > > > > > > P. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Regards, > > > > > > Christian. > > > > > > > > > > > > > > > > > > > > > > > > > > Replace the call to dma_fence_is_signaled() with > > > > > > > nouveau_fence_base_is_signaled(). > > > > > > > > > > > > > > Cc: <sta...@vger.kernel.org> # 4.10+, precise commit not > > > > > > > to > > > > > > > be > > > > > > > determined > > > > > > > Signed-off-by: Philipp Stanner <pha...@kernel.org> > > > > > > > --- > > > > > > > drivers/gpu/drm/nouveau/nouveau_fence.c | 2 +- > > > > > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > > > > > > > > > diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.c > > > > > > > b/drivers/gpu/drm/nouveau/nouveau_fence.c > > > > > > > index 7cc84472cece..33535987d8ed 100644 > > > > > > > --- a/drivers/gpu/drm/nouveau/nouveau_fence.c > > > > > > > +++ b/drivers/gpu/drm/nouveau/nouveau_fence.c > > > > > > > @@ -274,7 +274,7 @@ nouveau_fence_done(struct > > > > > > > nouveau_fence > > > > > > > *fence) > > > > > > > nvif_event_block(&fctx->event); > > > > > > > spin_unlock_irqrestore(&fctx->lock, > > > > > > > flags); > > > > > > > } > > > > > > > - return dma_fence_is_signaled(&fence->base); > > > > > > > + return test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, > > > > > > > &fence- > > > > > > > > > > > > > > > > > > > > > > > base.flags); > > > > > > > > > > > > > > > > > > > > > > } > > > > > > > > > > > > > > static long > > > > > > > > > > > > > > > > > > > > > > > > > > > > >
