Re: [RFC 02/17] dma-fence: basic lockdep annotations

[Daniel Vetter]

On Tue, May 12, 2020 at 09:09:52AM -0300, Jason Gunthorpe wrote:
> On Tue, May 12, 2020 at 10:59:29AM +0200, Daniel Vetter wrote:
> > diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> > index 6802125349fb..d5c0fd2efc70 100644
> > +++ b/drivers/dma-buf/dma-fence.c
> > @@ -110,6 +110,52 @@ u64 dma_fence_context_alloc(unsigned num)
> >  }
> >  EXPORT_SYMBOL(dma_fence_context_alloc);
> >  
> > +#ifdef CONFIG_LOCKDEP
> > +struct lockdep_map	dma_fence_lockdep_map = {
> > +	.name = "dma_fence_map"
> > +};
> > +
> > +bool dma_fence_begin_signalling(void)
> > +{
> 
> Why is this global? I would have expected it to be connected to a
> single fence?

It's the same rules for all fences, since they can be shared across
drivers in various ways. Lockdep usually achieves that with a static
variable hidden in the macro, but that doesn't work if you have lots of
different ways from different drivers to create a dma_fence. Hence the
unique global one that we explicitly allocate.

We have similar stuff for the global dma_resv_lock ww_mutex class, just
there it's a bit more standardized and hidden behind a neat macro. But
really lockdep needs global lockdep_maps or it doesn't work.

> It would also be alot nicer if this was some general lockdep feature,
> not tied to dmabuf. This exact problem also strikes anyone using
> completions, for instance, and the same solution should be
> applicable??

There was:

https://lwn.net/Articles/709849/

It even got merged, and seems to have worked. Unfortunately (and I'm not
entirely clear on the reasons) it was thrown out again, so we can't use
it. That means wait_event/wake_up dependencies need to be manually
annotated, like e.g. flush_work() already is. flush_work is more or less
where I've stolen this idea from, with some adjustements and tricks on top
to make it work for dma_fence users.

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch