Gaetano Mendola <mendola@gmail.com> writes:
On 07/19/2010 03:35 PM, Beman Dawes wrote:
It will be Wednesday before I can start pulling the 1.44 beta together. In the meantime, does anyone have any serious issues we need to tackle before the beta?
Ticket 2330 (https://svn.boost.org/trac/boost/ticket/2330) has a proposed patch to solve a showstopper issue related to interrupt threads, waiting on same condition, not working. The issue is there at least since 1.41. Anthony Williams stated that the patch isn't sufficient but at same time he didn't specify why. That patch applied proved to solve the problem I'm experiencing in my application and on the submitter's patch one.
The patch substitutes one race condition for other problems. If the patch is applied, one race condition is this: 1. Thread A calls wait() 2. Thread A sets the mutex and condvar in the interruption checker 3. Thread A calls pthread_cond_wait 4. Thread B calls interrupt() on thread A 5. Thread B gets the mutex and condvar for thread A 6. Thread C notifies thread A 7. Thread A wakes from the wait 8. Thread A DESTROYS THE MUTEX AND CONDVAR AS NO LONGER NEEDED 9. Thread B TRIES TO LOCK THE MUTEX AND NOTIFY THE CONDVAR => UNDEFINED BEHAVIOUR Also there is a potential for deadlock: 1. Thread A locks mutex M 2. Thread A calls wait with a lock on mutex M 3. Thread A sets the mutex and condvar in the interruption checker 4. Thread A calls pthread_cond_wait, which unlocks M 5. Thread B locks mutex M 6. Thread B calls interrupt() on thread A 7. Thread B gets the mutex (which is M) and condvar for thread A 8. Thread B tries to lock the mutex M => deadlock with itself This deadlock can of course also occur with other threads holding the mutex, if thread B holds a mutex the other thread needs I have thought about this problem lots, and decided that you cannot safely interrupt a condition variable wait on POSIX unless you have a mutex tied directly to each condition variable (as with condition_variable_any). I'm therefore not sure how to approach this --- either every boost::condition_variable has an extra pthread_mutex_t inside it, or interruption is limited to waits on boost::condition_variable_any. Anthony -- Author of C++ Concurrency in Action http://www.stdthread.co.uk/book/ just::thread C++0x thread library http://www.stdthread.co.uk Just Software Solutions Ltd http://www.justsoftwaresolutions.co.uk 15 Carrallack Mews, St Just, Cornwall, TR19 7UL, UK. Company No. 5478976