当只对变量进行inc/decrementing时，互斥是必需的吗?-Isamutexnecessarywhenonlyinc/decrementingavariable?

4  

You need to synchronize the access to the variables if the operations you are performing are not atomic. Consider a scenario of a single variable a having initial value of, say, 5. Now we have two threads T1 and T2, both want to increment it. How increment operation is broken down, in case it is not atomic (just an example, it could be some other way)?

如果正在执行的操作不是原子操作，则需要同步对变量的访问。考虑一个单变量a的初始值为5的场景。现在我们有两个线程T1和T2，都想增加它。如何分解递增操作，以防它不是原子操作(只是一个例子，它可以是其他的方法)?

1) Read a value into a temp location.
2) Increment and write the temp location back into a.

1)将一个值读入一个临时位置。2)递增并将临时位置写回a中。

Now consider a scenario T1 is performing this operation first, and then pre-empted by T2 after completing step 1 only:

现在考虑一种情形T1先执行这个操作，然后在完成第1步之后，在T2之前抢先一步:

T1: 1) Read a into temp1 => temp1=5
T2: 1) Read a into temp2 => temp2=5
T2: 2) Write temp2+1 into a => a=6
T1: 2) Write temp1+1 into a => a=6

1)将a读入temp1= > temp1=5 T2: 1)将a读入temp2= > temp2=5 T2: 2)将temp2+1写入a= > =6 T1: 2)将temp1+1写入a= > a=6

So the final value of a will be 6 and not 7 as you would expect.

a的最终值是6而不是7。

9  

The universal rule of concurrency is this: If a memory location is accessed concurrently in a non-atomic way, and not all accesses are reads, then the accesses must be ordered. Ordering can be imposed either by serializing access through locking a mutex, or by some lower-level operation like an ordered atomic access or an ordered fence.

并发的通用规则是:如果以非原子方式并发访问内存位置，并且不是所有访问都被读取，那么访问必须被排序。可以通过锁定互斥对象来序列化访问，也可以通过一些低级操作(如有序原子访问或有序围栏)来强制排序。

The only time you're allowed to have unordered accesses if all of the accesses are reads. E.g. multiple threads can read a (non-atomic) constant without ordering.

如果所有访问都是读操作，那么唯一一次允许有无序访问。例如，多个线程可以读取(非原子的)常数而不进行排序。

0  

If you have multiple threads updating global data then you should have mutex. You should use mutex in both cases in read and write. Only reading constant data have no issue.

如果您有多个线程更新全局数据，那么您应该有互斥对象。您应该在读写两种情况下都使用互斥锁。只读取常量数据没有问题。

If you avoid to use the mutex you will not get desired result because other threads will not get proper value. Have look on this When should we use mutex ?

如果您避免使用互斥对象，您将不会得到所需的结果，因为其他线程将得不到适当的值。看看这个，我们什么时候应该使用互斥?

Have look on example.

看例子。