What is the difference between semaphore and spinlock

I'll first describe difference between binary semaphore mutex and spin lock. Spin locks perform a busy wait - i. Hence if the thread will try to acquire blocked resource it will be suspended till it will be avaible for it. Semaphore is a lock that is allowed to be used multiple known from initialization number of times - for example 3 threads are allowed to simultainusly hold the resource but no more.

Spinlocks are used in an interrupt context, where sleeping is not allowed. This simplicity creates a small and fast lock. Spinlock is used if and only if you are pretty certain that your expected result will happen very shortly, before your thread's execution slice time expires. Example: In device driver module, The driver writes "0" in hardware Register R0 and now it needs to wait for that R0 register to become 1. This is generally quick in micro seconds. There is absolutely no reason for a user application to spin.

It doesn't make sense. You are going to spin for some event to happen and that event needs to be completed by another user level application which is never guaranteed to happen within quick time frame. So, I will not spin at all in user mode. I better to sleep or mutexlock or semaphore lock in user mode. From what is the difference between spin locks and semaphores? Both manage a limited resource. I'll first describe difference between binary semaphore mutex and spin lock.

Spin locks perform a busy wait - i. On the other hand mutex behave more like:. Hence if the thread will try to acquire blocked resource it will be suspended till it will be avaible for it. Semaphore is a lock that is allowed to be used multiple known from initialization number of times - for example 3 threads are allowed to simultainusly hold the resource but no more.

Locking in Linux. Spin lock wait until the process releases a lock and then acquires a lock. Semaphore is sleeping lock i. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Spinlock versus Semaphore Ask Question. Asked 13 years, 1 month ago.

Active 2 years, 6 months ago. Viewed 76k times. When would we use a semaphore over a spin-lock? Add a comment. Active Oldest Votes. Spinlock and semaphore differ mainly in four things: 1.

Insofar, one can consider a lock a special case of a semaphore with a maximum value of 1. Damon Damon Blocking mutexes and semaphores are not only useful in single-thread environments but also if there is oversubscription, that is, the number of threads a program or multiple programs sharing the system creates is higher than the number of hardware resources.

In these cases, blocking your thread allows the others to be able to use CPU time in a useful manner. In addition, if hardware supports hyperthreading, the other thread could make use of the execution units that are being used to perform the idle loop. Jonathan Leffler Jonathan Leffler k gold badges silver badges bronze badges. I'd like to second how important it is not to use spinlocks on single threaded systems.

They are a the ticked to priority inversion problems. And trust me: You don't want to debug these kind of bugs. What do you mean exactly? Amigable: by definition, a spinlock means that the current thread on the CPU is waiting for something else to release the locked object. If the only active thing that can change the lock is the current CPU, the lock will not be freed by spinning.

But spinning when nothing else can release the lock is not very sensible - you might as well yield the CPU to another process now as wait to be preempted. I may very well be wrong, but I was under the impression that a re-entrant single CPU Linux kernel may interrupt a running spin lock. Save Article. Improve Article.

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