select: Fix indefinitely sleeping task in poll_schedule_timeout()
A task can end up indefinitely sleeping in do_select() -> poll_schedule_timeout() when the following race happens: TASK1 (thread1) TASK2 TASK1 (thread2) do_select() setup poll_wqueues table with 'fd' write data to 'fd' pollwake() table->triggered = 1 closes 'fd' thread1 is waiting for poll_schedule_timeout() - sees table->triggered table->triggered = 0 return -EINTR loop back in do_select() But at this point when TASK1 loops back, the fdget() in the setup of poll_wqueues fails. So now so we never find 'fd' is ready for reading and sleep in poll_schedule_timeout() indefinitely. Treat an fd that got closed as a fd on which some event happened. This makes sure cannot block indefinitely in do_select(). Another option would be to return -EBADF in this case but that has a potential of subtly breaking applications that excercise this behavior and it happens to work for them. So returning fd as active seems like a safer choice. Suggested-by: Linus Torvalds <torvalds@linux-foundation.org> CC: stable@vger.kernel.org Signed-off-by: Jan Kara <jack@suse.cz> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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