std: use an event-flag-based thread parker on SOLID

[joboet]

Mutex and Condvar are being replaced by more efficient implementations, which need thread parking themselves (see #93740). Therefore, the generic Parker needs to be replaced on all platforms where the new lock implementation will be used, which, after #96393, are SOLID, SGX and Hermit (more PRs coming soon).

SOLID, conforming to the μITRON specification, has event flags, which are a thread parking primitive very similar to Parker. However, they do not make any atomic ordering guarantees (even though those can probably be assumed) and necessitate a system call even when the thread token is already available. Hence, this Parker, like the Windows parker, uses an extra atomic state variable.

I future-proofed the code by wrapping the event flag in a WaitFlag structure, as both SGX and Hermit can share the Parker implementation, they just have slightly different primitives (SGX uses signals and Hermit has a thread blocking API).

@kawadakk I assume you are the target maintainer? Could you test this for me?

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[rust-highfive]

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[joboet]

I noticed this could be using a semaphore, which might be more efficient, as the system does not need to compare the bitflags...

[kawadakk]

The following code deadlocks under this implementation:

let thread0 = std::thread::current();
std::thread::spawn(move || {
    thread0.unpark();
});

std::thread::park_timeout(std::time::Duration::from_millis(50)); // <--- blocks indefinitely here

Reconstructed execution sequence:

st      Thread 0                            Thread 1 
--------------------------------------------------------------------------
 0      park_timeout:
-1          st.fetch_sub(1)     // 0
            twai_flg:
                [yield]
                                            unpark:
 1                                              st.swap(1)  // -1
                                                set_flg:
                                                    [wake up thread 0]

                [resume]

 0          st.swap(0)          // 1
            wai_flg
            [deadlock]

[kawadakk]

The memory orderings can be relaxed to Acquire and Release like the futex-based implementation.

The rest looks good to me.

[joboet]

The memory orderings can be relaxed to Acquire and Release like the futex-based implementation.

Do we actually make any atomic ordering guarantees? It says so in the futex parker, but I can't find it in the documentation of unpark. Otherwise the ordering could be Relaxed.

[kawadakk]

It doesn't appear to be documented explicitly anywhere, but the common use cases of [un]park (alternating between calling park and checking an unblocking condition in a loop) and the past issues (e.g., #53366) seem to suggest that the requirement is to either form a synchronizes-with edge in token passing, or have a no-op unpark. If unpark blockingly consumed a token without forming a synchronizes-with edge, it would be completely useless, since it wouldn't propagate the change in the unblocking condition to the destination thread.

[joboet]

@kawadakk Ah, I understand now. Thank you for all the help!

[joboet]

ping @m-ou-se: I think all issues are fixed.

[m-ou-se]

@bors r+

[bors]

📌 Commit caff723 has been approved by m-ou-se