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The main thread sleeps 10ms, then expects > futex_wake() to wake one waiter. So the test assumes that the waiter > reaches futex_wait() within 10ms, and that the main thread runs again > within 30ms. > > A busy machine breaks both. The built-in locktorture at times holds a > spinlock for 100ms with preemption off. With it, private_futex failed in > about 0.5% of the runs in 8-CPU VMs, on v7.4-rc1. > Timestamps showed two cases. The waiter started up to 75ms late, after > the wake. Or the main thread woke up to 105ms late, after the waiter > had timed out. > > Fix this by retrying futex_wake() every millisecond until it wakes the > waiter, for up to 5s. Give the waiter a timeout of 10s, so that it > outlasts the retries. The tests then wait for the waiter, instead of > assuming when it runs. > > With this patch, private_futex passed 6000 runs of 6000 in the same VMs, > and the three tests 2000 runs of 2000. Self-nack. That's flaky as well. A bounded retry can still fail. This needs a proper synchronization flag. Will respin a v2. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com