linux-kernel - Re: [PATCH rcu] srcu: Guarantee non-negative return value from srcu_read

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Message-ID: <CAEf4BzZU4ysQznVEctzijCUyuwN0TQXsxg_C16v3mmhUOzspjQ@mail.gmail.com>
Date: Mon, 21 Oct 2024 16:50:44 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: paulmck@...nel.org
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com, 
	rostedt@...dmis.org, peterz@...radead.org, andrii@...nel.org
Subject: Re: [PATCH rcu] srcu: Guarantee non-negative return value from srcu_read_lock()

On Mon, Oct 21, 2024 at 3:13 PM Paul E. McKenney <paulmck@...nel.org> wrote:
>
> For almost 20 years, the int return value from srcu_read_lock() has
> been always either zero or one.  This commit therefore documents the
> fact that it will be non-negative.
>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Andrii Nakryiko <andrii@...nel.org
>
> diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> index bab1dae3f69e6..512a8c54ba5ba 100644
> --- a/include/linux/srcu.h
> +++ b/include/linux/srcu.h
> @@ -238,13 +238,14 @@ void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor);
>   * a mutex that is held elsewhere while calling synchronize_srcu() or
>   * synchronize_srcu_expedited().
>   *
> - * The return value from srcu_read_lock() must be passed unaltered
> - * to the matching srcu_read_unlock().  Note that srcu_read_lock() and
> - * the matching srcu_read_unlock() must occur in the same context, for
> - * example, it is illegal to invoke srcu_read_unlock() in an irq handler
> - * if the matching srcu_read_lock() was invoked in process context.  Or,
> - * for that matter to invoke srcu_read_unlock() from one task and the
> - * matching srcu_read_lock() from another.
> + * The return value from srcu_read_lock() is guaranteed to be
> + * non-negative.  This value must be passed unaltered to the matching
> + * srcu_read_unlock().  Note that srcu_read_lock() and the matching
> + * srcu_read_unlock() must occur in the same context, for example, it is
> + * illegal to invoke srcu_read_unlock() in an irq handler if the matching
> + * srcu_read_lock() was invoked in process context.  Or, for that matter to
> + * invoke srcu_read_unlock() from one task and the matching srcu_read_lock()
> + * from another.

For uprobe work I'm using __srcu_read_lock() and __srcu_read_unlock().
Presumably the same non-negative index will be returned/consumed there
as well, right? Can we add a blurb to that effect for them as well?

Otherwise LGTM, thanks!

Acked-by: Andrii Nakryiko <andrii@...nel.org>

>   */
>  static inline int srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp)
>  {