Bug 24741 – Stop the world logic is invalid and might stop a thread mid critical section.

A thread enter critical section by taking the criticalRegionLock and setting its m_isInCriticalRegion flag. A thread leaves critical section in a similar way, taking the criticalRegionLock and unsetting its m_isInCriticalRegion flag. When stopping the world, the criticalRegionLock is taken during the whole process. For each thread, m_isInCriticalRegion before suspending. If it is set, then the lock is temporarily released and we wait to give the thread an opportunity to exit its critical section, and then then lock is taken again, and we check again if the thread is in its critical section, etc... To suspend a thread, we send a signal using pthread_kill , which is asynchronous. This means that by the time we try to suspend thread n + 1, thread n might not be suspended yet. If thread n + 1 is is in its critical section, we'll release criticalRegionLock, allowing thread n to enter a critical section before it had the chance to suspend. In practice, critical section are rare enough this doesn't happen much, but a carefully designed test case can trigger the race condition, and potentially an attacker feeding malicious input to an application.

Resolved by https://github.com/dlang/dmd/commit/cbbc96f5b13319331c80292822843feea61851fd