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Merge pull request #663 from tlkh/patch-1

Proposed improvements to explanation of spinlock
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0xAX 8 months ago
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      SyncPrim/linux-sync-3.md

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@@ -15,9 +15,9 @@ Introduction to the semaphores in the Linux kernel

So, what is it `semaphore`? As you may guess - `semaphore` is yet another mechanism for support of thread or process synchronization. The Linux kernel already provides implementation of one synchronization mechanism - `spinlocks`, why do we need in yet another one? To answer on this question we need to know details of both of these mechanisms. We already familiar with the `spinlocks`, so let's start from this mechanism.

The main idea behind `spinlock` concept is a lock which will be acquired for a very short time. We can't sleep when a lock acquired by a process or thread, because other processes wait us. [Context switch](https://en.wikipedia.org/wiki/Context_switch) is not not allowed because [preemption](https://en.wikipedia.org/wiki/Preemption_%28computing%29) is disabled to avoid [deadlocks](https://en.wikipedia.org/wiki/Deadlock).
`spinlock` creates a lock which will be acquired to protect a shared resource from being modified by more than one process. As a result, other processes that try to acquire the current lock get stopped (aka "spin-in-place" or busy waiting). [Context switch](https://en.wikipedia.org/wiki/Context_switch) is not allowed because [preemption](https://en.wikipedia.org/wiki/Preemption_%28computing%29) is disabled to avoid [deadlocks](https://en.wikipedia.org/wiki/Deadlock). As a result, `spinlock` should only be used if the lock will only be acquired for a very short period of time, otherwise amount of busy waiting accumulated by other processes results in extremely inefficient operation. For locks that need to be acquired for a relatively long period of time, we turn to `semaphore`.

In this way, [semaphores](https://en.wikipedia.org/wiki/Semaphore_%28programming%29) is a good solution for locks which may be acquired for a long time. In other way this mechanism is not optimal for locks that acquired for a short time. To understand this, we need to know what is `semaphore`.
[semaphores](https://en.wikipedia.org/wiki/Semaphore_%28programming%29) is a good solution for locks which may be acquired for a long time. In other way this mechanism is not optimal for locks that acquired for a short time. To understand this, we need to know what is `semaphore`.

As usual synchronization primitive, a `semaphore` is based on a variable. This variable may be incremented or decremented and it's state will represent ability to acquire lock. Notice that value of the variable is not limited to `0` and `1`. There are two types of `semaphores`:


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