5a66a7b0fc
A typical ussage of localcount_drain looks like this: mutex_enter(&mtx); item = remove_from_list(); pserialize_perform(psz); localcount_drain(&item->localcount, &cv, &mtx); mutex_exit(&mtx); This sequence can cause a deadlock which happens for example on the following situation: - Thread A calls localcount_drain which calls xc_broadcast after releasing a specified mutex - Thread B enters the sequence and calls pserialize_perform with holding the mutex while pserialize_perform also calls xc_broadcast - Thread C (xc_thread) that calls an xcall callback of localcount_drain tries to hold the mutex xc_broadcast of thread B doesn't start until xc_broadcast of thread A finishes, which is a feature of xcall(9). This means that pserialize_perform never complete until xc_broadcast of thread A finishes. On the other hand, thread C that is a callee of xc_broadcast of thread A sticks on the mutex. Finally the threads block each other (A blocks B, B blocks C and C blocks A). A possible fix is to serialize executions of the above sequence by another mutex, but adding another mutex makes the code complex, so fix the deadlock by another way; the fix is to release the mutex before pserialize_perform and instead use a condvar to prevent pserialize_perform from being called simultaneously. Note that the deadlock has happened only if NET_MPSAFE is enabled.