RFC: get_dma_buf_unless_zero ?

[Thomas Hellstrom]

Anyone else but me that feels such a function could be useful?

My main use-case is that it would resolve the mutual refcounting problem:

1) drm buffer object caches a dma_buf pointer which it refcounts
2) The dma-buf holds a refcount to the buffer.

This is resolved today by having the user-space visible part of the 
drm-buffer holding the refcount to the dma_buf. When user-space closes 
the drm-buffer, the reference goes away, and eventually the buffer is 
freed, when all external dma-buf users are done with the dma-buf

However, this also means that the dma-buf remains for the buffer 
lifetime even when there are no external users, which bugs me a bit.

This can be resolved by viewing the drm buffer as a lookup structure 
that doesn't hold a refcount to the dma-buf, but that means that the 
lookup structure (buffer) would need to share locks with the dma-buf 
implementation, unless we have a get_dma_buf_unless_zero, which means we 
can use locks local to the lookup structure, the drm buffer.
(See the last part of the kref documentation for a detailed discussion 
of this).

Now I don't think keeping the dma_buf for the drm buffer lifetime is a 
HUGE problem, but I just wanted to get people's views of this.

Thanks,
Thomas
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