Concurrency bugs that stem from schedule-dependent branches are hard to understand and debug, because their root causes imply not only different event orderings, but also changes in the control-flow between failing and non-failing executions. We present Cortex: a system that helps exposing and understanding concurrency bugs that result from schedule-dependent branches, without relying on information from failing executions. Cortex preemptively exposes failing executions by perturbing the order of events and controlflow behavior in non-failing schedules from production runs of a program. By leveraging this information from production runs, Cortex synthesizes executions to guide the search for failing schedules. Production-guided search helps cope with the large execution search space by targeting failing executions that are similar to observed non-failing executions. Evaluation on popular benchmarks shows that Cortex is able to expose failing schedules with only a few perturbations to non-failing executions, and takes a practical amount of time.

We present Symbiosis: a concurrency debugging technique based on novel differential schedule projections (DSPs). A DSP shows the small set of memory operations and data-flows responsible for a failure, as well as a reordering of those elements that avoids the failure. To build a DSP, Symbiosis first generates a full, failing, multithreaded schedule via thread path profiling and symbolic constraint solving. Symbiosis selectively reorders events in the failing schedule to produce a non-failing, alternate schedule. A DSP reports the ordering and data-flow differences between the failing and non-failing schedules. Our evaluation on buggy real-world software and benchmarks shows that, in practical time, Symbiosis generates DSPs that both isolate the small fraction of event orders and data-flows responsible for the failure, and show which event reorderings prevent failing. In our experiments, DSPs contain 81% fewer events and 96% less data-flows than the full failure-inducing schedules. Moreover, by allowing developers to focus on only a few events, DSPs reduce the amount of time required to find a valid fix.