Ticket #3448 (closed Bugs: fixed)
interprocess_condition (emulated) can exit with inconsistent m_num_waiters value
|Reported by:||Zachariah L Young <zachariah.l.young@…>||Owned by:||igaztanaga|
I describe this from the point of view of the 1.39.0 source code, but the problem still exists in the boost development trunk as of today.
There is a set of conditions where a process can manage to enter do_timed_wait, increment m_num_waiters, and exit without decrementing it.
Sequence of events:
We join our hero, Process A (P_A), in boost/interprocess/sync/emulation/interprocess_condition.hpp.
P_A is executing a do_timed_wait(true, lock, abs_time) call, and is spinning at the while loop at line 124.
tout_enabled == true, and abs_time is a microsecond in the future (about to expire but hasn't yet).
Process B, P_A's trusty sidekick, sends a notify_all on the conditional, breaking P_A out of the while loop at line 124.
abs_time arrives (ie, P_A got to line 149 with microsec_clock::universal_time() >= abs_time and timed_out = false).
With these conditions, P_A gets to line 163 and calls the constructor for scoped_lock.
P_A jumps to boost/interprocess/sync/scoped_lock.hpp line 114.
P_A executes mp_mutex->timed_lock(abs_time) at line 115.
P_A jumps to boost/interprocess/sync/emulation/interprocess_condition.hpp line 49.
P_A takes a reading of now at line 56.
P_A finds that (now >= abs_time) at line 58 and is sent packing with a return value of false.
P_A arrives back in boost/interprocess/sync/emulation/interprocess_condition.hpp on line 163.
P_A gets to line 171 and finds lock is false. He panics! He sets timed_out to true and unlock_enter_mut to true, but in his haste to break out of evil Dr. while(1)'s clutches, he forgot to atomically decrement m_num_waiters!
Maniacal laughter can be heard behind him as he tries in vein to acquire the lock on line 214.
"You fool! You fell into my trap!", shouts Dr. while(1). "Process B grabbed that very lock and attempted to free you again! He is at line 56 of this very header file, waiting for a call from you that will never come, and he's holding your precious lock! Your deadlock is complete! HAHAHAHAHAHAH!!"