Friday, December 25, 2009

RAII mutex needed -- no guaranteed unlock at thread death

(See other posts on thread death)

Update: RAII mutex introduces data inconsistency and is not always a good thing.
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Mutex is shared by multiple stacks so can't live on stack, but in a special semaphore memory area. According to [[OO MT using c++]], some semaphores (mutex locks) are shared among PROCESSES, so must live outside the address space of any process. When a process (or thread) dies, it could die owning the semaphore, blocking other processes indefinitely.

Because mutex release is manual, not by default, you should consider RAII, such as boost scoped_lock. See other posts on RAII mutex.

http://publib.boulder.ibm.com/infocenter/iseries/v6r1m0/topic/apis/concep26.htm says

If a thread is the owner of one or more exclusive write locks acquired by pthread_rwlock_wrlock(), pthread_rwlock_trywrlock(), or pthread_rwlock_timedwrlock_np(), when that thread terminates, the exclusive write locks are not automatically released by the system. This is an error in the application and indicates that the data associated with the lock is in an inconsistent state. If another thread attempts to get a shared read or exclusive write lock on the lock, that thread blocks forever.

http://www.yolinux.com/TUTORIALS/LinuxTutorialPosixThreads.html echos

"When a thread terminates, the mutex does not unless explicitly unlocked. Nothing happens by default."

[[c++ cookbook]] echos "If an operating system thread is killed, it will not release its locks..."

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