Am 01.12.21 um 09:23 schrieb Thomas Hellström (Intel):
[SNIP]
Jason and I came up with a deep dive iterator for his use case, but I
think we don't want to use that any more after my dma_resv rework.
In other words when you need to create a new dma_fence_array you
flatten
out the existing construct which is at worst case
dma_fence_chain->dma_fence_array->dma_fence.
Ok, Are there any cross-driver contract here, Like every driver
using a
dma_fence_array need to check for dma_fence_chain and flatten like
above?
So far we only discussed that on the mailing list but haven't made
any documentation for that.
OK, one other cross-driver pitfall I see is if someone accidently
joins two fence chains together by creating a fence chain unknowingly
using another fence chain as the @fence argument?
That would indeed be illegal and we should probably add a WARN_ON() for
that.
The third cross-driver pitfall IMHO is the locking dependency these
containers add. Other drivers (read at least i915) may have defined
slightly different locking orders and that should also be addressed if
needed, but that requires a cross driver agreement what the locking
orders really are. Patch 1 actually addresses this, while keeping the
container lockdep warnings for deep recursions, so at least I think
that could serve as a discussion starter.
No, drivers should never make any assumptions on that.
E.g. when you need to take a look from a callback you must guarantee
that you never have that lock taken when you call any of the dma_fence
functions. Your patch breaks the lockdep annotation for that.
What we could do is to avoid all this by not calling the callback with
the lock held in the first place.
/Thomas
Oh, and a follow up question:
If there was a way to break the recursion on final put() (using the
same basic approach as patch 2 in this series uses to break
recursion in enable_signaling()), so that none of these containers
did require any special treatment, would it be worth pursuing? I
guess it might be possible by having the callbacks drop the
references rather than the loop in the final put. + a couple of
changes in code iterating over the fence pointers.
That won't really help, you just move the recursion from the final
put into the callback.
How do we recurse from the callback? The introduced fence_put() of
individual fence pointers
doesn't recurse anymore (at most 1 level), and any callback recursion
is broken by the irq_work?
Yeah, but then you would need to take another lock to avoid racing with
dma_fence_array_signaled().
I figure the big amount of work would be to adjust code that iterates
over the individual fence pointers to recognize that they are rcu
protected.
Could be that we could solve this with RCU, but that sounds like a lot
of churn for no gain at all.
In other words even with the problems solved I think it would be a
really bad idea to allow chaining of dma_fence_array objects.
Christian.
Thanks,
/Thomas
