How is the access to field synchronized

Question

I'm looking at this snippet from seastar tutorial:

#include <seastar/core/coroutines.hh>
#include <seastar/coroutine/parallel_for_each.hh>

seastar::future<bool> all_exist(std::vector<sstring> filenames) {
    bool res = true;
    co_await seastar::coroutine::parallel_for_each(filenames, [&res] (const seastar::sstring& name) -> seastar::future<> {
        res &= co_await seastar::file_exists(name);
    });
    co_return res;
}

I'm learning advanced C++, I suspect there is a race in this code. The &= operator is in fact two operations, read and write, and it's not atomic. The lambda captures res by reference and multiple threads modify it in parallel. This scenario can happen:

• both threads A and B read true from res
• thread A writes false
• thread B, not seeing the write by A, overwrites it back to true

Is the above scenario possible? If not, why not?

Answer 1

The seastar::coroutine::parallel_for_each method, according to documentation, processes all elements in a single shard (i.e., a single thread). So no synchronization is necessary.

The parallelism comes from executing using coroutines - multiple file_exists calls are initiated and wait for their return values in parallel, but the return values are processed by a single thread.

The name is a bit confusing, elsewhere "parallel" means "multiple threads", e.g. here or here.