[Intel-gfx] [PATCH 0/5] dma-fence, i915: Stop allowing SLAB_TYPESAFE_BY_RCU for dma_fence

Fri Jun 11 09:33:12 UTC 2021

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 at amd.com> wrote:
> > > Am 10.06.21 um 22:42 schrieb Daniel Vetter:
> > > > On Thu, Jun 10, 2021 at 10:10 PM Jason Ekstrand <jason at jlekstrand.net> wrote:
> > > > > On Thu, Jun 10, 2021 at 8:35 AM Jason Ekstrand <jason at jlekstrand.net> wrote:
> > > > > > On Thu, Jun 10, 2021 at 6:30 AM Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> > > > > > > On Thu, Jun 10, 2021 at 11:39 AM Christian König
> > > > > > > <christian.koenig at amd.com> 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