Re: [PATCH rcu] srcu: Guarantee non-negative return value from srcu_read_lock()

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Oct 21, 2024 at 08:30:43PM -0700, Paul E. McKenney wrote:
> Thoughts?

Thanks Paul!

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

> ------------------------------------------------------------------------
> 
> commit 4433b7d3785d8d2a700f5ed5ca234c64bc63180e
> Author: Paul E. McKenney <paulmck@kernel.org>
> Date:   Mon Oct 21 15:09:39 2024 -0700
> 
>     srcu: Guarantee non-negative return value from srcu_read_lock()
>     
>     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, and does the same for the underlying
>     __srcu_read_lock().
>     
>     [ paulmck: Apply Andrii Nakryiko feedback. ]
>     
>     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>     Cc: Peter Zijlstra <peterz@infradead.org>
>     Acked-by: Andrii Nakryiko <andrii@kernel.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.
>   */
>  static inline int srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp)
>  {
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 07147efcb64d3..ae17c214e0de5 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -738,7 +738,8 @@ EXPORT_SYMBOL_GPL(srcu_check_read_flavor);
>  /*
>   * Counts the new reader in the appropriate per-CPU element of the
>   * srcu_struct.
> - * Returns an index that must be passed to the matching srcu_read_unlock().
> + * Returns a guaranteed non-negative index that must be passed to the
> + * matching __srcu_read_unlock().
>   */
>  int __srcu_read_lock(struct srcu_struct *ssp)
>  {