On 04/12/2017 18:13, Peter Maydell wrote: > Hi; in https://bugs.linaro.org/show_bug.cgi?id=3259 comment 27 > Stuart provides backtraces of a deadlock in user-mode in the RCU > code. > > How should this deadlock be broken ?
Summing up: - fork thread: has mmap_lock, waits for rcu_sync_lock - RCU thread: has rcu_sync_lock, waits for rcu_read_(un)lock - another CPU thread: in RCU critical section, waits for mmap_lock Probably the best solution is to add start_exclusive/end_exclusive respectively at the beginning and the end of fork_start and fork_end. This is safer in general, as it ensures that the disappeared child threads were quiescent. In fact, I wonder if fork_start/fork_end still need to "take all mutexes" (in pthread_atfork style) if we do start_exclusive/end_exclusive in fork_start and fork_end(0). You don't even need to reinitialize the mutexes, meaning that mmap_fork_start and mmap_fork_end should go as well. The list of locks that are "assured not taken" within start_exclusive/end_exclusive (currently: rcu_read_lock, tb_lock, mmap_lock) should probably be documented in fork_start/fork_end. Thanks, Paolo
