Thanks T.J for the reply!! On 5/4/2024 4:43 AM, T.J. Mercier wrote: > It looks like a similar conclusion about epoll was reached at: > https://lore.kernel.org/all/a87d7ef8-2c59-4dc5-ba0a-b821d1effc72@xxxxxxx/ > I am unaware of this discussion. Thanks... > I agree with Christian that it should not be possible for the file to > be freed while inside dma_buf_poll. Aside from causing problems in > dma_buf_poll, ep_item_poll itself would have issues dereferencing the > freed file pointer. > Not sure about my understanding: ep_item_poll() always call the ->poll() interface with a stable 'struct file' because of ep->mtx. This lock ensures that: a) If eventpoll_release_file() get the ep->mtx first, ->poll() corresponds to the epitem(target file) will never be called, because it is removed from the rdlist. b) If ep_send_events() get the ep->mtx() first, ->poll() will get called with a stable 'struct file', __but the refcount(->f_count) of a file can be zero__. I am saying that this is stable because the 'struct file' contents are still valid till we are in ->poll(). Can you/Christian help me with what I am missing here to say that ->poll() is receiving stale 'struct file*', please? And, If you are convinced with above, I think, It should have been the responsibility of ->poll() implementation to have taken refcount on a file that is going to be still valid even after ->poll() exits. Incase of dma_buf_poll() implementation, it took the refcount on a file that is not going to be valid once the dma_buf_poll() exits(because of mentioned race with the freeing of the 'struct file*'). So, in dma_buf_poll(), Should we be using atomic_long_inc_not_zero() based implementation to take the refcount on a file? Thanks, Charan
