is assignment operator '=' atomic?

Question

I'm implementing Inter-Thread Communication using global variable.

//global var
volatile bool is_true = true;

//thread 1
void thread_1()
{
    while(1){
        int rint = rand() % 10;
        if(is_true) {
            cout << "thread_1: "<< rint <<endl;  //thread_1 prints some stuff
            if(rint == 3)
                is_true = false;  //here, tells thread_2 to start printing stuff
        }
    }
}

//thread 2
void thread_2()
{
    while(1){
        int rint = rand() % 10;
        if(! is_true) {  //if is_true == false
            cout << "thread_1: "<< rint <<endl;  //thread_2 prints some stuff
            if(rint == 7)  //7
                is_true = true;  //here, tells thread_1 to start printing stuff
        }
    }
}

int main()
{
    HANDLE t1 = CreateThread(0,0, thread_1, 0,0,0);
    HANDLE t2 = CreateThread(0,0, thread_2, 0,0,0);
    Sleep(9999999);
    return 0;
}

Question

In the code above, I use a global var volatile bool is_true to switch printing between thread_1 and thread_2.

I wonder whether it is thread-safe to use assignment operation here?

Answer 1

This code is not guaranteed to be thread-safe on Win32, since Win32 guarantees atomicity only for properly-aligned 4-byte and pointer-sized values. bool is not guaranteed to be one of those types. (It is typically a 1-byte type.)

For those who demand an actual example of how this could fail:

Suppose that bool is a 1-byte type. Suppose also that your is_true variable happens to be stored adjacent to another bool variable (let's call it other_bool), so that both of them share the same 4-byte line. For concreteness, let's say that is_true is at address 0x1000 and other_bool is at address 0x1001. Suppose that both values are initially false, and one thread decides to update is_true at the same time another thread tries to update other_bool. The following sequence of operations can occur:

• Thread 1 prepares to set is_true to true by loading the 4-byte value containing is_true and other_bool. Thread 1 reads 0x00000000.
• Thread 2 prepares to set other_bool to true by loading the 4-byte value containing is_true and other_bool. Thread 2 reads 0x00000000.
• Thread 1 updates the byte in the 4-byte value corresponding to is_true, producing 0x00000001.
• Thread 2 updates the byte in the 4-byte value corresponding to other_bool, producing 0x00000100.
• Thread 1 stores the updated value to memory. is_true is now true and other_bool is now false.
• Thread 2 stores the updated value to memory. is_true is now false and other_bool is now true.

Observe that at the end this sequence, the update to is_true was lost, because it was overwritten by thread 2, which captured an old value of is_true.

It so happens that x86 is very forgiving of this type of error because it supports byte-granular updates and has a very tight memory model. Other Win32 processors are not as forgiving. RISC chips, for example, often do not support byte-granular updates, and even if they do, they usually have very weak memory models.

Answer 2

no, its not..... you need to use a locking primitive of some sort. Depending on platform, you can either use boost ones, or if going native windows, something like InterlockedCompareExchange.

In fact in your situation you might what to use some of the thread safe event mechanisims so you can 'signal' your other thread to start doing what you want.

Answer 3

On all modern processors, you can assume that reads and writes of naturally aligned native types are atomic. As long as the memory bus is at least as wide as the type being read or written, the CPU reads and writes these types in a single bus transaction, making it impossible for other threads to see them in a half-completed state. On x86 and x64 there, is no guarantee that reads and writes larger than eight bytes are atomic. This means that 16-byte reads and writes of streaming SIMD extension (SSE) registers, and string operations, might not be atomic.

Reads and writes of types that are not naturally aligned—for instance, writing DWORDs that cross four-byte boundaries—are not guaranteed to be atomic. The CPU may have to do these reads and writes as multiple bus transactions, which could allow another thread to modify or see the data in the middle of the read or write.

Answer 4

The thread safeness of this piece of code does not depend on atomicity of the assignment. Both thread routines work strictly in turn. There is no race condition: thread_1 will output stuff until getting certain random number after which it will leave the 'output section' and let the other thread work in it. There are a couple of things worth noting though:

• rand() function may be not thread-safe (not the problem in the code given here though)
• you should not use Win32 function CreateThread(), especially when you are using CRT libraly functions which (potentially) utilize global variables. Use _beginthreadex() instead.