When is a mutex needed

This int may be set and read by either thread. Do I need a mutex around it if it's only an int? Is it really possible that the integer could be "half-read" at some point, causing an error?

Before I go into any of this, let me say up front that "volatile" is NOT the solution. In case anybody wants to claim that. If the type is atomic which int usually is, provided it is properly aligned in memory , then you do not strictly need a mutex to protect access to it. However, there are pitfalls if you don't use one. The biggest is the fact that the compiler may cache the value of the variable in a register, causing a thread to not see updates made to the variable.

In most modern operating systems, a thread exists within a process — that is, a single process may contain multiple threads. A mutex object is a synchronization object whose state is set to signaled when it is not owned by any thread, and nonsignaled when it is owned.

Only one thread at a time can own a mutex object, whose name comes from the fact that it is useful in coordinating mutually exclusive access to a shared resource. A computer lock is a lock on a computer. A lock may be used on a computer's casing to prevent it from being opened to remove or swap out parts, or install unauthorized components. And a lock may be used on a computer to disallow it from being turned on or used without a physical key.

In computer science, a lock or mutex from mutual exclusion is a synchronization mechanism for enforcing limits on access to a resource in an environment where there are many threads of execution. A lock is designed to enforce a mutual exclusion concurrency control policy. A race condition is an undesirable situation that occurs when a device or system attempts to perform two or more operations at the same time, but because of the nature of the device or system, the operations must be done in the proper sequence to be done correctly.

Threads and its types in Operating System. Spinning with a CAS is useful in some situations, and I did it on one of my projects. There are a couple things to note:. They are genrally only useful when you have threads that can a lot of processing without needing to otherwise wait. We had some special threads like this on our finance project; they were pure processing, all IO was done on other threads.

Thanks for simple, straight forward explanation. Which system call is invoked when there is contention for locks in user space? Can i assume it is futex call?

As you said, this is only done when there is contention, or when something voluntarily wants to pass control to the kernel. Non-contentious locking is handling in user space, with a compare-and-swap or similar instruction.

Hi, i had a doubt. If a process p1 unlocks mutex and enters critical section, then can another process lock mutex and enter critical section? Which mutex lock implementation is fastest on commonly implemented OS designs?

There is no absolute fastest design. Which lock performs the best depends a lot on how it is being used, and what needs to be accomplished. In all cases, contention is the biggest performance drawback. My questions are: 1. Is this waiting mechanism as you said, implemented in the OS level? I guess so, because wait have to be done in OS level. If it is, then why bother any test-and-set in user space? How is the waiting mechanism implemented? You are commenting using your WordPress. You are commenting using your Google account.

You are commenting using your Twitter account. You are commenting using your Facebook account. Notify me of new comments via email. Notify me of new posts via email. Menu About Edaqa What is Programming? Efficiency How does a mutex work? What does it cost? Like this: Like Loading Categories: Efficiency , Programming Tagged as: concurrency , concurrent programming , contention , cpu memory , critical data , efficiency , fundamental operations , linux , lock , mutex , Programming.

How to fail a programming interview. Stop waving the wand of magic numbers. How does any thread will put into waiting state when the lock is not available? This is OS dependent. Thanks for the simple and concise article. When we know that lock section may be bit big I assume that price to pay is the context switch, but we can never avoid it, as we need CPU to have some useful work rather than waiting.

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