Re: [Linaro-mm-sig] Re: [PATCH] dma-fence: allow dma fence to have their own lock

[Sergey Senozhatsky]

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.