> The signal handler calls Py_AddPendingCall() which blocks on acquiring > "pending_lock". It blocks in taking the mutex, not on waiting for the condition variable. Otherwise it wouldn't block (microseconds = 0). I think the solution is to protect signal_handler() against reentrancy: diff --git a/Modules/signalmodule.c b/Modules/signalmodule.c --- a/Modules/signalmodule.c +++ b/Modules/signalmodule.c @@ -185,10 +185,12 @@ signal_handler(int sig_num) Handlers[sig_num].tripped = 1; /* Set is_tripped after setting .tripped, as it gets cleared in PyErr_CheckSignals() before .tripped. */ - is_tripped = 1; - Py_AddPendingCall(checksignals_witharg, NULL); - if (wakeup_fd != -1) - write(wakeup_fd, "\0", 1); + if (!is_tripped) { + is_tripped = 1; + Py_AddPendingCall(checksignals_witharg, NULL); + if (wakeup_fd != -1) + write(wakeup_fd, "\0", 1); + } } #ifndef HAVE_SIGACTION