Question

I have a parent and a worker thread that share a bool flag and a std::vector. The parent only reads (i.e., reads the bool or calls my_vector.empty()); the worker only writes.

My questions:

• Do I need to mutex protect the bool flag?

• Can I say that all bool read/writes are inherently atomic operations? If you say Yes or No, where did you get your information from?

• I recently heard about GCC Atomic-builtin. Can I use these to make my flag read/writes atomic without having to use mutexes? What is the difference? I understand Atomic builtins boil down to machine code, but even mutexes boil down to CPU’s memory barrier instructions right? Why do people call mutexes an “OS-level” construct?

• Do I need to mutex protect my std::vector? Recall that the worker thread populates this vector, whereas the parent only calls empty() on it (i.e., only reads it)

• I do not believe mutex protection is necessary for either the bool or the vector. I rationalize as follows, “Ok, if I read the shared memory just before it was updated.. thats still fine, I will get the updated value the next time around. More importantly, I do not see why the writer should be blocked while the reading is reading, because afterall, the reader is only reading!”

If someone can point me in the right direction, that would be just great. I am on GCC 4.3, and Intel x86 32-bit.
Thanks a lot!

Answer 1

Do I need to mutex protect the bool flag?

Not necessarily, an atomic instruction would do. By atomic instruction I mean a compiler intrinsic function that a) prevents compiler reordering/optimization and b) results in atomic read/write and c) issues an appropriate memory fence to ensure visibility between CPUs (not necessary for current x86 CPUs which employ MESI cache coherency protocol). Similar to gcc atomic builtins.

Can I say that all bool read/writes are inherently atomic operations? If you say Yes or No, where did you get your information from?

Depends on the CPU. For Intel CPUs – yes. See Intel® 64 and IA-32 Architectures Software Developer’s Manuals.

I recently heard about GCC Atomic-builtin. Can I use these to make my flag read/writes atomic without having to use mutexes? What is the difference? I understand Atomic builtins boil down to machine code, but even mutexes boil down to CPU’s memory barrier instructions right? Why do people call mutexes an “OS-level” construct?

The difference between atomics and mutexes is that the latter can put the waiting thread to sleep until the mutex is released. With atomics you can only busy-spin.

Do I need to mutex protect my std::vector? Recall that the worker thread populates this vector, whereas the parent only calls empty() on it (i.e., only reads it)

You do.

I do not believe mutex protection is necessary for either the bool or the vector. I rationalize as follows, “Ok, if I read the shared memory just before it was updated.. thats still fine, I will get the updated value the next time around. More importantly, I do not see why the writer should be blocked while the reading is reading, because afterall, the reader is only reading!”

Depending on the implementation, vector.empty() may involve reading two buffer begin/end pointers and subtracting or comparing them, hence there is a chance that you read a new version of one pointer and an old version of another one without a mutex. Surprising behaviour may ensue.