2017-07-27 Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>:
> Quoting Gustavo Padovan (2017-07-27 20:03:53)
> > From: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxx>
> > ---
> > 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
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