How to make sure C multithreading program read the latest value from main memory?

Question

Let's say thread 1(executed in Core 0) updates a global variable, and the updated global value is cached in Core 0's L1 cache(not flushed to the main memory). Then thread 2 starts to execute in Core 3, and it tries to read the global variable, and read it from the main memory(since it doesn't have the cached value), so thread 2 is reading a outdated value.

I know in C you can use volatile to force the compilier do not read the value form CPU registers, which means that volatile varaible will get its value from cache or main memory. In my above scenario, even if I declare the global variable with volatile, the latest value will still be cached in L1 cache, the main memory still has an old value which will be read by thread 2. So how can we fix this issue? or maybe my understanding is wrong, using volatile will make the variable updated in main memory directly so everytime you try to read/write a volatile variable, you read/write it from/to the main memory directly?

Answer 1

To some extent, people noting that the premise of your question is flawed is a reasonable answer. In general, this happens rarely if at all, and is usually indistinguishable from a race condition.

However yes it can happen. See for example memory barriers which are a great example of how such a condition (albeit due to OOO execution etc.) can occur.

That being said, what you're looking for to make sure the specific occurrence you've noted cannot happen is called a "cache flush". This can be genuinely important on ARM/ARM64 processors where separate data and instruction caches exist, but it's also a good habit to get into for data that is passed between threads this way. You can also check out the __builtin___clear_cache c compiler builtin which performs a similar task. Hopefully one of these will help you get to the bottom of your problem.

However, most likely you're not running into a caching issue, and a race condition is far more likely to be arising. If memory barriers/cache flushes don't fix your issue, audit your code very carefully for raciness.