On Fri, Jun 11, 2021 at 8:55 AM Christian König <christian.koenig@xxxxxxx> wrote: > > Am 10.06.21 um 22:42 schrieb Daniel Vetter: > > On Thu, Jun 10, 2021 at 10:10 PM Jason Ekstrand <jason@xxxxxxxxxxxxxx> wrote: > >> On Thu, Jun 10, 2021 at 8:35 AM Jason Ekstrand <jason@xxxxxxxxxxxxxx> wrote: > >>> On Thu, Jun 10, 2021 at 6:30 AM Daniel Vetter <daniel.vetter@xxxxxxxx> wrote: > >>>> On Thu, Jun 10, 2021 at 11:39 AM Christian König > >>>> <christian.koenig@xxxxxxx> wrote: > >>>>> Am 10.06.21 um 11:29 schrieb Tvrtko Ursulin: > >>>>>> On 09/06/2021 22:29, Jason Ekstrand wrote: > >>>>>>> We've tried to keep it somewhat contained by doing most of the hard work > >>>>>>> to prevent access of recycled objects via dma_fence_get_rcu_safe(). > >>>>>>> However, a quick grep of kernel sources says that, of the 30 instances > >>>>>>> of dma_fence_get_rcu*, only 11 of them use dma_fence_get_rcu_safe(). > >>>>>>> It's likely there bear traps in DRM and related subsystems just waiting > >>>>>>> for someone to accidentally step in them. > >>>>>> ...because dma_fence_get_rcu_safe apears to be about whether the > >>>>>> *pointer* to the fence itself is rcu protected, not about the fence > >>>>>> object itself. > >>>>> Yes, exactly that. > >>> The fact that both of you think this either means that I've completely > >>> missed what's going on with RCUs here (possible but, in this case, I > >>> think unlikely) or RCUs on dma fences should scare us all. > >> Taking a step back for a second and ignoring SLAB_TYPESAFE_BY_RCU as > >> such, I'd like to ask a slightly different question: What are the > >> rules about what is allowed to be done under the RCU read lock and > >> what guarantees does a driver need to provide? > >> > >> I think so far that we've all agreed on the following: > >> > >> 1. Freeing an unsignaled fence is ok as long as it doesn't have any > >> pending callbacks. (Callbacks should hold a reference anyway). > >> > >> 2. The pointer race solved by dma_fence_get_rcu_safe is real and > >> requires the loop to sort out. > >> > >> But let's say I have a dma_fence pointer that I got from, say, calling > >> dma_resv_excl_fence() under rcu_read_lock(). What am I allowed to do > >> with it under the RCU lock? What assumptions can I make? Is this > >> code, for instance, ok? > >> > >> rcu_read_lock(); > >> fence = dma_resv_excl_fence(obj); > >> idle = !fence || test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags); > >> rcu_read_unlock(); > >> > >> This code very much looks correct under the following assumptions: > >> > >> 1. A valid fence pointer stays alive under the RCU read lock > >> 2. SIGNALED_BIT is set-once (it's never unset after being set). > >> > >> However, if it were, we wouldn't have dma_resv_test_singnaled(), now > >> would we? :-) > >> > >> The moment you introduce ANY dma_fence recycling that recycles a > >> dma_fence within a single RCU grace period, all your assumptions break > >> down. SLAB_TYPESAFE_BY_RCU is just one way that i915 does this. We > >> also have a little i915_request recycler to try and help with memory > >> pressure scenarios in certain critical sections that also doesn't > >> respect RCU grace periods. And, as mentioned multiple times, our > >> recycling leaks into every other driver because, thanks to i915's > >> choice, the above 4-line code snippet isn't valid ANYWHERE in the > >> kernel. > >> > >> So the question I'm raising isn't so much about the rules today. > >> Today, we live in the wild wild west where everything is YOLO. But > >> where do we want to go? Do we like this wild west world? So we want > >> more consistency under the RCU read lock? If so, what do we want the > >> rules to be? > >> > >> One option would be to accept the wild-west world we live in and say > >> "The RCU read lock gains you nothing. If you want to touch the guts > >> of a dma_fence, take a reference". But, at that point, we're eating > >> two atomics for every time someone wants to look at a dma_fence. Do > >> we want that? > >> > >> Alternatively, and this what I think Daniel and I were trying to > >> propose here, is that we place some constraints on dma_fence > >> recycling. Specifically that, under the RCU read lock, the fence > >> doesn't suddenly become a new fence. All of the immutability and > >> once-mutability guarantees of various bits of dma_fence hold as long > >> as you have the RCU read lock. > > Yeah this is suboptimal. Too many potential bugs, not enough benefits. > > > > This entire __rcu business started so that there would be a lockless > > way to get at fences, or at least the exclusive one. That did not > > really pan out. I think we have a few options: > > > > - drop the idea of rcu/lockless dma-fence access outright. A quick > > sequence of grabbing the lock, acquiring the dma_fence and then > > dropping your lock again is probably plenty good. There's a lot of > > call_rcu and other stuff we could probably delete. I have no idea what > > the perf impact across all the drivers would be. > > The question is maybe not the perf impact, but rather if that is > possible over all. > > IIRC we now have some cases in TTM where RCU is mandatory and we simply > don't have any other choice than using it. Adding Thomas Hellstrom. Where is that stuff? If we end up with all the dma_resv locking complexity just for an oddball, then I think that would be rather big bummer. > > - try to make all drivers follow some stricter rules. The trouble is > > that at least with radeon dma_fence callbacks aren't even very > > reliable (that's why it has its own dma_fence_wait implementation), so > > things are wobbly anyway. > > > > - live with the current situation, but radically delete all unsafe > > interfaces. I.e. nothing is allowed to directly deref an rcu fence > > pointer, everything goes through dma_fence_get_rcu_safe. The > > kref_get_unless_zero would become an internal implementation detail. > > Our "fast" and "lockless" dma_resv fence access stays a pile of > > seqlock, retry loop and an a conditional atomic inc + atomic dec. The > > only thing that's slightly faster would be dma_resv_test_signaled() > > > > - I guess minimally we should rename dma_fence_get_rcu to > > dma_fence_tryget. It has nothing to do with rcu really, and the use is > > very, very limited. > > I think what we should do is to use RCU internally in the dma_resv > object but disallow drivers/frameworks to mess with that directly. > > In other words drivers should use one of the following: > 1. dma_resv_wait_timeout() > 2. dma_resv_test_signaled() > 3. dma_resv_copy_fences() > 4. dma_resv_get_fences() > 5. dma_resv_for_each_fence() <- to be implemented > 6. dma_resv_for_each_fence_unlocked() <- to be implemented > > Inside those functions we then make sure that we only save ways of > accessing the RCU protected data structures. > > This way we only need to make sure that those accessor functions are > sane and don't need to audit every driver individually. Yeah better encapsulation for dma_resv sounds like a good thing, least for all the other issues we've been discussing recently. I guess your list is also missing the various "add/replace some more fences" functions, but we have them already. > I can tackle implementing for the dma_res_for_each_fence()/_unlocked(). > Already got a large bunch of that coded out anyway. When/where do we need ot iterate over fences unlocked? Given how much pain it is to get a consistent snapshot of the fences or fence state (I've read the dma-buf poll implementation, and it looks a bit buggy in that regard, but not sure, just as an example) and unlocked iterator sounds very dangerous to me. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
