Compare and swap - What if 2 processors execute locking simultaneous?

Question

I read about CAS in https://en.wikipedia.org/wiki/Compare-and-swap, and got some doubts:

• 1. Even though a single lock operation is implemented in a single instruction, but if 2 threads run on 2 different processors, then the 2 instruction could happen at the same time. Isn't that a race condition?
• 2. I saw following sentence in <Linux Kernel Development> 3rd page 168.

  because a process can execute on only one processor at a time

  I doubt that, not sure does it means what it literally says. What if the process has multiple threads, can't they run on multiple processors at a time?

Anyone help to explain these doubts? Thanks.

Answer 1

The cpu has a cache for memory, typically 64 bytes of size per so-called cache line. It will do stuff with respect to chunks of that size. In particular, when doing lock cmpxchg or similar things, the hardware thread you execute this on will negotiate exclusive access to the 64-byte portion of memory with other threads. And that's why it works.

In general, you want to read this book: https://www.kernel.org/pub/linux/kernel/people/paulmck/perfbook/perfbook.html

This particular bit is explained on page 21.

Regarding the LKD quote, there is no context provided. It is safe to assume they meant threads and were updating a thread-local counter.