Am 04.03.20 um 17:41 schrieb Jason Ekstrand:
On Wed, Mar 4, 2020 at 10:27 AM Jason Ekstrand <jason@xxxxxxxxxxxxxx> wrote:
On Wed, Mar 4, 2020 at 2:34 AM Christian König <christian.koenig@xxxxxxx> wrote:
Am 03.03.20 um 20:10 schrieb Jason Ekstrand:
On Thu, Feb 27, 2020 at 2:28 AM Christian König
<christian.koenig@xxxxxxx> wrote:
[SNIP]
For reference see what dance is necessary in the dma_fence_chain_release
function to avoid that:
/* Manually unlink the chain as much as possible to avoid
recursion
* and potential stack overflow.
*/
while ((prev = rcu_dereference_protected(chain->prev, true))) {
....
It took me quite a while to figure out how to do this without causing
issues. But I don't see how this would be possible for dma_fence_array.
Ah, I see the issue now! It hadn't even occurred to me that userspace
could use this to build up an infinite recursion chain. That's nasty!
Yeah, when I first stumbled over it it was like why the heck is my code
crashing in an interrupt handler?
Realizing that this is stack corruption because of the long chain we
constructed was quite an enlightenment.
And then it took me even longer to fix it :)
I'll give this some more thought and see if can come up with
something clever.
Here's one thought: We could make dma_fence_array automatically
collapse any arrays it references and instead directly reference their
fences. This way, no matter how much the client chains things, they
will never get more than one dma_fence_array. Of course, the
difficulty here (answering my own question) comes if they ping-pong
back-and-forth between something which constructs a dma_fence_array
and something which constructs a dma_fence_chain to get
array-of-chain-of-array-of-chain-of-... More thought needed.
Condensing the fences into a larger array can certainly work, yes.
Answering my own questions again... I think the
array-of-chain-of-array case is also solvable.
For array-of-chain, we can simply add all unsignaled dma_fences in the
chain to the array. The array won't signal until all of them have
which is exactly the same behavior as if we'd added the chain itself.
Yeah, that should work. Probably best to implement something like a
cursor to walk all fences in the data structure.
For chain-of-array, we can add all unsignaled dma_fences in the array
to the same point in the chain. There may be some fiddling with the
chain numbering required here but I think we can get it so the chain
won't signal until everything in the array has signaled and we get the
same behavior as if we'd added the dma_fence_array to the chain.
Well as far as I can see this won't work because it would break the
semantics of the timeline sync.
But I think I know a different way which should work: A dma_fence_chain
can still contain a dma_fence_array, only the other way around is
forbidden. Then we create the cursor functionality in such a way that it
allows us to deep dive into the data structure and return all containing
fences one by one.
I can prototype that if you want, shouldn't be more than a few hours of
hacking anyway.
Regards,
Christian.
In both cases, we end up with either a single array or a single and
destruction doesn't require recursion. Thoughts?
--Jason
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