[PATCH] dma-buf/sw_sync: hold a fence reference when check if it signaled

[Gustavo Padovan]

2017-07-27 Chris Wilson <chris at chris-wilson.co.uk>:

> Quoting Gustavo Padovan (2017-07-27 20:03:53)
> > From: Gustavo Padovan <gustavo.padovan at collabora.com>
> > 
> > If userspace already dropped its own reference by closing the sw_sync
> > fence fd we might end up in a deadlock where
> > dma_fence_is_signaled_locked() will trigger the release of the fence a
> > thus try to hold the lock to remove the fence from the list.
> 
> So the issue here is that call to dma_fence_is_signaled_lock() is
> triggering the unreference?

Exactly. I'll say that explicitely in the commit message.

>  
> > We need to grab a reference to the fence before calling into this chain if
> > we want to avoid this issue.
> > 
> > Cc: Chris Wilson <chris at chris-wilson.co.uk>
> > Signed-off-by: Gustavo Padovan <gustavo.padovan at collabora.com>
> > ---
> >  drivers/dma-buf/sw_sync.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/dma-buf/sw_sync.c b/drivers/dma-buf/sw_sync.c
> > index af1bc84..8291434 100644
> > --- a/drivers/dma-buf/sw_sync.c
> > +++ b/drivers/dma-buf/sw_sync.c
> > @@ -144,11 +144,16 @@ static void sync_timeline_signal(struct sync_timeline *obj, unsigned int inc)
> >         obj->value += inc;
> >  
> >         list_for_each_entry_safe(pt, next, &obj->pt_list, link) {
> > -               if (!dma_fence_is_signaled_locked(&pt->base))
> > +               dma_fence_get(&pt->base);
> 
> This would need to be dma_fence_get_rcu() to avoid grabbing the fence
> when its refcount has hit 0.
> 
> > +               if (!dma_fence_is_signaled_locked(&pt->base)) {
> > +                       dma_fence_put(&pt->base);
> >                         break;
> > +               }
> >  
> >                 list_del_init(&pt->link);
> >                 rb_erase(&pt->node, &obj->pt_tree);
> 
> But if I understand correctly, we just need to unlink first, then
> signal.
> 
> list_for_each_entry_safe() {
> 	if (!timeline_fence_signaled(&pt->base))
> 		break;
> 
> 	list_del_init(&pt->link);
> 	rb_erase(&pt->node, &obj->pt_tree);
> 
> 	dma_fence_signal_locked(&pt->base);
> }
> 
> The challenge is in writing the comment to explain the open-coding.

That is cleaner and doesn't need the get/put dance. I'll come up with a
comment to explain it.

Gustavo