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