On Fri, Jun 11, 2021 at 09:42:07AM +0200, Christian König wrote: > Am 11.06.21 um 09:20 schrieb Daniel Vetter: > > 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. > > This is during buffer destruction. See the call to dma_resv_copy_fences(). Ok yeah that's tricky. The way solved this in i915 is with a trylock and punting to a worker queue if the trylock fails. And the worker queue would also be flushed from the shrinker (once we get there at least). So this looks fixable. > But that is basically just using a dma_resv function which accesses the > object without taking a lock. The other one I've found is the ghost object, but that one is locked fully. > > > > - 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. > > This is to make implementation of the other functions easier. Currently they > basically each roll their own loop implementation which at least for > dma_resv_test_signaled() looks a bit questionable to me. > > Additionally to those we we have one more case in i915 and the unlocked > polling implementation which I agree is a bit questionable as well. Yeah, the more I look at any of these lockless loop things the more I'm worried. 90% sure the one in dma_buf_poll is broken too. > My idea is to have the problematic logic in the iterator and only give back > fence which have a reference and are 100% sure the right one. > > Probably best if I show some code around to explain what I mean. My gut feeling is that we should just try and convert them all over to taking the dma_resv_lock. And if there is really a contention issue with that, then either try to shrink it, or make it an rwlock or similar. But just the more I read a lot of the implementations the more I see bugs and have questions. Maybe at the end a few will be left over, and then we can look at these individually in detail. Like the ttm_bo_individualize_resv situation. -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
