Re: [patch 4/8] futex: Add support for unlocking robust futexes

[André Almeida]

Em 16/03/2026 14:13, Thomas Gleixner escreveu:

[...]

> --- a/kernel/futex/waitwake.c
> +++ b/kernel/futex/waitwake.c
> @@ -150,12 +150,32 @@ void futex_wake_mark(struct wake_q_head
>   }
>   
>   /*
> + * If requested, clear the robust list pending op and unlock the futex
> + */
> +static bool futex_robust_unlock(u32 __user *uaddr, unsigned int flags, void __user *pop)
> +{
> +	if (!(flags & FLAGS_UNLOCK_ROBUST))
> +		return true;
> +
> +	/* First unlock the futex. */
> +	if (put_user(0U, uaddr))
> +		return false;
> +

On glibc code, the futex unlock happens atomically:

    atomic_exchange_release (&mutex->__data.__lock, 0)

Is OK to do it unatomically?

I couldn't find a race condition given that the only thread that should 
be able to write to the futex address must be the lock owner anyways, 
but I don't know why userspace does it atomically in the first place.