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There are reports for hang in __check_pf: https://github.com/JoeDog/siege/issues/4 It is reproducible only under specific configurations: 1. Large number of cores (>= 64) and large number of threads (> 3X of the number of cores) with long lived socket connection. 2. Low power (frequency) mode. 3. Power management is enabled. While holding lock, __check_pf calls make_request which calls __sendto and __recvmsg. Since __sendto and __recvmsg are cancellation points, lock held by __check_pf won't be released and can cause deadlock when thread cancellation happens in __sendto or __recvmsg. Add a cancellation cleanup handler for __check_pf to unlock the lock when cancelled by another thread. This fixes BZ #20975 and the siege hang issue.