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.
