[Linaro-mm-sig] [PATCH 2/2] dma-buf: Fix dma_resv_test_signaled.

[Daniel Vetter]

On Fri, Oct 15, 2021 at 02:56:59PM +0200, Christian König wrote:
> 
> 
> Am 15.10.21 um 14:52 schrieb Maarten Lankhorst:
> > Op 15-10-2021 om 14:07 schreef Christian König:
> > > Am 15.10.21 um 13:57 schrieb Maarten Lankhorst:
> > > > Commit 7fa828cb9265 ("dma-buf: use new iterator in dma_resv_test_signaled")
> > > > accidentally forgot to test whether the dma-buf is actually signaled, breaking
> > > > pretty much everything depending on it.
> > > NAK, the dma_resv_for_each_fence_unlocked() returns only unsignaled fences. So the code is correct as it is.
> > That seems like it might cause some unexpected behavior when that function is called with one of the fence locks held, if it calls dma_fence_signal().
> > 
> > Could it be changed to only test the signaled bit, in which case this patch would still be useful?
> 
> That's exactly what I suggested as well, but Daniel was against that because
> of concerns around barriers.

I don't want open-coded bitmask tests, because the current code we have in
dma-fence.c is missing barriers, and that doesn't get better if we spread
that all around. But if you want this then wrap it in some static inline
in dma-fence.h or so, that's fine. Just not open-coded outside of these
files, like i915-gem code does a lot (which imo is just plain a disaster).

> > Or at least add some lockdep annotations, that fence->lock might be taken. So any hangs would at least be easy to spot with lockdep.
> 
> That should be trivial doable.

might_lock is trivial to add, but it's more complicated. The spinlock is
provided by the fence code, which means there's lots of different lockdep
classes. A might_lock on fence->lock is better than nothing, but maybe not
good enough.

What we might need are a few more pieces:
- a fake dma-fence spinlock lockdep key, maybe call it dma_fence_lock_key
  or so.
- in dma_fence_init we lock dma_fence_lock_key, and then might_lock the
  actual spinlock passed as an argument. This establishes dependencies
  from that fake lock to all real fence spinlocks
- anywhere we need a might lock we take dma_fence_lock_key instead

The potential issue here is that this might result in lockdep splats in
cases where fences somehow naturally nest (maybe drm/sched job fence vs hw
fence). So perhaps too much.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch