Re: [PATCH 3/3] selftests/futex: fix the issue of abnormal test results caused by thread timing

[Yuwen Chen <ywen.chen@foxmail.com>]

on Fri, 12 Dec 2025 12:30:20 +0800, Licay wrote:
> 1. Avoid Parsing /proc
>    The current approach uses get_thread_state() to read /proc/$pid/status, which isn't very reliable.
>    A better way would be to have waiterfn directly signal when it's ready using atomic operations.

I haven't found a better way to check whether a process has entered the
sleep state without using the proc interface for the time being. The kernel
probably doesn't provide a system call to obtain whether a process has
entered the sleep state.

> 2. Use Atomic Counting Instead of Polling Thread State
>    Before entering futex_wait, waiterfn can atomically increment a counter. The parent thread then just waits for this counter to reach the expected value.
>    This is much simpler and avoids the overhead of checking /proc repeatedly.

Using atomic instructions does not guarantee that the process is waiting in futex_wait.

> 3. Use Standard Atomic Types
>    Replace the custom READ_ONCE/WRITE_ONCE macros with standard <stdatomic.h> types like atomic_int.
>    It's cleaner and more portable across different platforms.

Only two operations, atomic_set and atomic_read, are used here. Using
WRITE_ONCE and READ_ONCE can avoid introducing too many dependencies.

> Here's the basic idea:
> - Add a global atomic_int ready_count variable
> - In waiterfn: atomic_fetch_add(&ready_count, 1) right before futex_wait()
> - Parent thread: spin-wait until atomic_load(&ready_count) reaches the expected value

Similarly, using atomic_inc before futex_wait does not guarantee that
the thread has reached the futex_wait execution point.

Thank you very much for your reply.