[PATCH] dma-buf/sync_file: Always increment refcount when merging fences.

[Gustavo Padovan]

2016-09-14 Chris Wilson <chris at chris-wilson.co.uk>:

> On Tue, Sep 13, 2016 at 04:24:27PM -0700, Rafael Antognolli wrote:
> > The refcount of a fence should be increased whenever it is added to a merged
> > fence, since it will later be decreased when the merged fence is destroyed.
> > Failing to do so will cause the original fence to be freed if the merged fence
> > gets freed, but other places still referencing won't know about it.
> > 
> > This patch fixes a kernel panic that can be triggered by creating a fence that
> > is expired (or increasing the timeline until it expires), then creating a
> > merged fence out of it, and deleting the merged fence. This will make the
> > original expired fence's refcount go to zero.
> > 
> > Signed-off-by: Rafael Antognolli <rafael.antognolli at intel.com>
> > ---
> > 
> > Sample code to trigger the mentioned kernel panic (might need to be executed a
> > couple times before it actually breaks everything):
> > 
> > static void test_sync_expired_merge(void)
> > {
> >        int iterations = 1 << 20;
> >        int timeline;
> >        int i;
> >        int fence_expired, fence_merged;
> > 
> >        timeline = sw_sync_timeline_create();
> > 
> >        sw_sync_timeline_inc(timeline, 100);
> >        fence_expired = sw_sync_fence_create(timeline, 1);
> >        fence_merged = sw_sync_merge(fence_expired, fence_expired);
> >        sw_sync_fence_destroy(fence_merged);
> > 
> >        for (i = 0; i < iterations; i++) {
> >                int fence = sw_sync_merge(fence_expired, fence_expired);
> > 
> >                igt_assert_f(sw_sync_wait(fence, -1) > 0,
> >                                     "Failure waiting on fence\n");
> >                sw_sync_fence_destroy(fence);
> >        }
> > 
> >        sw_sync_fence_destroy(fence_expired);
> > }
> > 
> >  drivers/dma-buf/sync_file.c | 7 ++-----
> >  1 file changed, 2 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/dma-buf/sync_file.c b/drivers/dma-buf/sync_file.c
> > index 486d29c..6ce6b8f 100644
> > --- a/drivers/dma-buf/sync_file.c
> > +++ b/drivers/dma-buf/sync_file.c
> > @@ -178,11 +178,8 @@ static struct fence **get_fences(struct sync_file *sync_file, int *num_fences)
> >  static void add_fence(struct fence **fences, int *i, struct fence *fence)
> >  {
> >  	fences[*i] = fence;
> > -
> > -	if (!fence_is_signaled(fence)) {
> > -		fence_get(fence);
> > -		(*i)++;
> > -	}
> > +	fence_get(fence);
> > +	(*i)++;
> >  }
> 
> I think you'll find it's the caller:
> 
> if (i == 0) {
> 	add_fence(fences, &i, a_fences[0]);
> 	i++;
> }
> 
> that does the unexpected.
> 
> This should be 
> 
> if (i == 0)
> 	fences[i++] = fence_get(a_fences[0]);

You are right. Otherwise we would miss the extra reference. i == 0 here
means that all fences are signalled.

> 
> 
> That ensures the sync_file inherits the signaled status without having
> to keep all fences.
> 
> I think there still seems to be a memory leak when calling
> sync_file_set_fence() here with i == 1.

I think that is something we discussed already, we don't hold an extra
ref there for i == 1 because it would leak the fence.

Gustavo