2017-07-27 Chris Wilson chris@chris-wilson.co.uk:

Quoting Gustavo Padovan (2017-07-27 20:03:53)

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From: Gustavo Padovan gustavo.padovan@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.

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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@chris-wilson.co.uk Signed-off-by: Gustavo Padovan gustavo.padovan@collabora.com

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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