On (22/06/01 14:45), Christian König wrote:
Am 31.05.22 um 04:51 schrieb Sergey Senozhatsky:
On (22/05/30 16:55), Christian König wrote:
Am 30.05.22 um 16:22 schrieb Sergey Senozhatsky:
[SNIP] So the `lock` should have at least same lifespan as the DMA fence that borrows it, which is impossible to guarantee in our case.
Nope, that's not correct. The lock should have at least same lifespan as the context of the DMA fence.
How does one know when it's safe to release the context? DMA fence objects are still transparently refcount-ed and "live their own lives", how does one synchronize lifespans?
Well, you don't.
If you have a dynamic context structure you need to reference count that as well. In other words every time you create a fence in your context you need to increment the reference count and every time a fence is release you decrement it.
OK then fence release should be able to point back to its "context" structure. Either a "private" data in dma fence or we need to "embed" fence into another object (refcounted) that owns the lock and provide dma fence ops->release callback, which can container_of() to the object that dma fence is embedded into.
I think you are suggesting the latter. Thanks for clarifications.
The limiting factor of this approach is that now our ops->release() is under the same "pressure" as dma_fence_put()->dma_fence_release() are. dma_fence_put() and dma_fence_release() can be called from any context, as far as I understand, e.g. IRQ, however our normal object ->release can schedule, we do things like synchronize_rcu() and so on. Nothing is impossible, just saying that even this approach is not 100% perfect and may need additional workarounds.
If you have a static context structure like most drivers have then you must make sure that all fences at least signal before you unload your driver. We still somewhat have a race when you try to unload a driver and the fence_ops structure suddenly disappear, but we currently live with that.
Hmm, indeed... I didn't consider fence_ops case.
Apart from that you are right, fences can live forever and we need to deal with that.
OK. I see.