Can a bool read/write operation be not atomic on x86? [duplicate]

Question

Say we have two threads, one is reading a bool in a loop and another can toggle it at certain times. Personally I think this should be atomic because sizeof(bool) in C++ is 1 byte and you don't read/write bytes partially but I want to be 100% sure.

So yes or no?

EDIT:

Also for future reference, does the same apply to int?

Answer 1

There are three separate issues that "atomic" types in C++11 address:

1. tearing: a read or write involves multiple bus cycles, and a thread switch occurs in the middle of the operation; this can produce incorrect values.

2. cache coherence: a write from one thread updates its processor's cache, but does not update global memory; a read from a different thread reads global memory, and doesn't see the updated value in the other processor's cache.

3. compiler optimization: the compiler shuffles the order of reads and writes under the assumption that the values are not accessed from another thread, resulting in chaos.

Using std::atomic<bool> ensures that all three of these issues are managed correctly. Not using std::atomic<bool> leaves you guessing, with, at best, non-portable code.

Answer 2

It all depends on what you actually mean by the word "atomic".

Do you mean "the final value will be updated in one go" (yes, on x86 that's definitely guaranteed for a byte value - and any correctly aligned value up to 64 bits at least), or "if I set this to true (or false), no other thread will read a different value after I've set it" (that's not quite such a certainty - you need a "lock" prefix to guarantee that).