id,summary,reporter,owner,description,type,status,milestone,component,version,severity,resolution,keywords,cc
3448,interprocess_condition (emulated) can exit with inconsistent m_num_waiters value,Zachariah L Young <zachariah.l.young@…>,Ion Gaztañaga,"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.

Bug:

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.

Boost 1.39.0

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!!""",Bugs,closed,Boost 1.45.0,interprocess,Boost 1.40.0,Problem,fixed,interprocess_condition,