In https://github.com/mongodb/mongo/blob/8e4b5670df9b9fe814e57cb5f3f8ee9407237b5a/src/mongo/util/signal_handlers_synchronous.cpp , the server defines signal-handlers for a variety of signals that can be synchronously generated, like SIGSEGV and SIGABRT. The signal-handling action for these signals is defined to be some version of logging a fatal error, collecting and logging a backtrace, and the exiting. Today, this includes some known signal-unsafe behavior, such as use of mutex as described in this comment. This can result in potential deadlocks when handling a signal.

It can block other signals from being effectively processed, because the mutex is held by another signal-handler invocation. This means that if an i.e. invariant fires in the server while another signal-handler has gotten stuck, the abort raised by the invariant will never be processed.