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2590740b08
The `num_idle` field of the shared state between BlockingPool worker threads can underflow in the following case: * state.num_idle == 2. * We spawn 2 new tasks into the BlockingPool. * Both idle worker threads are woken up. `state.num_idle` goes to 0. * The first worker thread wakes up and pulls a task from the queue. That task finishes very quickly so the worker thread pulls the second task from the queue before the second worker thread is scheduled. * The second worker thread is scheduled. It sees that `s.tasks.is_empty() == true` so it goes back to waiting on the Condvar. * The second worker thread's wait times out and it tries to decrement `state.num_idle` leading to underflow. Fix this by adding a `num_notified` field to the shared worker state. This field acts like a counter for the number of idle worker threads that have been woken up. When an idle thread is waiting on a Condvar, rather than checking if the task queue is empty, it will instead check if num_notified > 0. When an idle worker thread observes that num_notified > 0 it decrements it by 1 and then goes back to processing tasks from the queue. num_idle is only decremented when num_notified is 0. Change the num_idle decrement to a checked_sub so that we can catch it even when -Coverflow_checks=off. Also add a test for this case. This test consistently panics without the num_notified changes. BUG=none TEST=unit tests Change-Id: Ia1b348605e0d02415635cdd023db1c10201ab661 Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3139159 Tested-by: kokoro <noreply+kokoro@google.com> Reviewed-by: Richard Zhang <rizhang@google.com> Reviewed-by: Noah Gold <nkgold@google.com> Reviewed-by: Daniel Verkamp <dverkamp@chromium.org> Commit-Queue: Chirantan Ekbote <chirantan@chromium.org> |
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