Am 06.04.23 um 10:49 schrieb Asahi Lina:
On 06/04/2023 17.29, Christian König wrote:
Am 05.04.23 um 18:34 schrieb Asahi Lina:
A signaled scheduler fence can outlive its scheduler, since fences are
independently reference counted.
Well that is actually not correct. Schedulers are supposed to stay
around until the hw they have been driving is no longer present.
But the fences can outlive that. You can GPU render into an imported
buffer, which attaches a fence to it. Then the GPU goes away but the
fence is still attached to the buffer. Then you oops when you cat that
debugfs file...
No, exactly that's the point you wouldn't ops.
My use case does this way more often (since schedulers are tied to
UAPI objects), which is how I found this, but as far as I can tell
this is already broken for all drivers on unplug/unbind/anything else
that would destroy the schedulers with fences potentially referenced
on separate scanout devices or at any other DMA-BUF consumer.
Even if a GPU is hot plugged the data structures for it should only go
away with the last reference, since the scheduler fence is referencing
the hw fence and the hw fence in turn is referencing the driver this
shouldn't happen.
E.g. the reference was scheduler_fence->hw_fence->driver->scheduler.
It's up to drivers not to mess that up, since the HW fence has the
same requirements that it can outlive other driver objects, just like
any other fence. That's not something the scheduler has to be
concerned with, it's a driver correctness issue.
Of course, in C you have to get it right yourself, while with correct
Rust abstractions will cause your code to fail to compile if you do it
wrong ^^
In my particular case, the hw_fence is a very dumb object that has no
references to anything, only an ID and a pending op count. Jobs hold
references to it and decrement it until it signals, not the other way
around. So that object can live forever regardless of whether the rest
of the device is gone.
That is then certainly a bug. This won't work that way, and the timelime
name is just the tip of the iceberg here.
The fence reference count needs to keep both the scheduler and driver
alive. Otherwise you could for example unload the module and immediately
ops because your fence_ops go away.
Your use case is now completely different to that and this won't work
any more.
This here might just be the first case where that breaks.
This bug already exists, it's just a lot rarer for existing use
cases... but either way Xe is doing the same thing I am, so I'm not
the only one here either.
No it doesn't. You just have implemented the references differently than
they are supposed to be.
Fixing this one occasion here would mitigate that immediate ops, but
doesn't fix the fundamental problem.
Regards,
Christian.
~~ Lina
