It is both complicated and differs from Unix to Unix variant. In Linux, for example, a system called Futex (Short for Fast Userspace Mutex) is used. In this system an atomic increment and test operation is performed on the mutex variable in user space. If the result of the operation indicates that there was no … Read more
I was just going to edit this answer, but it doesn’t seem correct. So I’ll post my own… According to the Threads for C# page, which has a lot of synchronization tutorials, AutoResetEvent cannot be used for interprocess synchronization. However, a named EventWaitHandle can be used for interprocess synchronization. In the above page, visit the … Read more
Yes, there is indeed a race condition in the sample script. You can use bash’s noclobber option in order to get a failure in case of a race, when a different script sneaks in between the -f test and the touch. The following is a sample code-snippet (inspired by this article) that illustrates the mechanism: … Read more
It is not supported because the std::mutex implementation uses GetCurrentThreadId(). That’s a winapi function that is not supposed to be use in managed code since it might be running on a custom CLR host that doesn’t use threads to implement threading. This is the good kind of problem to have, it shows that you are … Read more
The spinlock mutex implementation looks okay to me. I think they got the definitions of acquire and release completely wrong. Here is the clearest explanation of acquire/release consistency models that I am aware of: Gharachorloo; Lenoski; Laudon; Gibbons; Gupta; Hennessy: Memory consistency and event ordering in scalable shared-memory multiprocessors, Int’l Symp Comp Arch, ISCA(17):15-26, 1990, … Read more
By older versions of the POSIX standard the first method with an initializer is only guaranteed to work with statically allocated variables, not when the variable is an auto variable that is defined in a function body. Although I have never seen a platform where this would not be allowed, even for auto variables, and … Read more
Atomic operations leverage processor support (compare and swap instructions) and don’t use locks at all, whereas locks are more OS-dependent and perform differently on, for example, Win and Linux. Locks actually suspend thread execution, freeing up cpu resources for other tasks, but incurring in obvious context-switching overhead when stopping/restarting the thread. On the contrary, threads … Read more
Pthreads has 3 different kinds of mutexes: Fast mutex, recursive mutex, and error checking mutex. You used a fast mutex which, for performance reasons, will not check for this error. If you use the error checking mutex on Linux you will find you get the results you expect. Below is a small hack of your … Read more
You need to tell the mutex to be process-shared when it’s inited: http://www.opengroup.org/onlinepubs/007908775/xsh/pthread_mutexattr_setpshared.html Note in particular, “The default value of the attribute is PTHREAD_PROCESS_PRIVATE”, meaning that accessing it from different processes is undefined behaviour.
As I understand this is covered in: 1.10 Multi-threaded executions and data races Para 5: The library defines a number of atomic operations (Clause 29) and operations on mutexes (Clause 30) that are specially identified as synchronization operations. These operations play a special role in making assignments in one thread visible to another. A synchronization … Read more