Re: [PATCH v2 1/8] sched: introduce for_each_process_rculock and for_each_thread_rculock

[Lorenzo Stoakes (ARM)]

On Mon, Sep 07, 2026 at 04:13:27PM +0800, Ye Liu wrote:
> From: Ye Liu <liuye@xxxxxxxxxx>
>
> Introduce for_each_process_rculock(), for_each_thread_rculock() and
> for_each_process_thread_rculock() macros that acquire the RCU read
> lock before the iteration starts and release it when the loop is left,
> so that the RCU read-side critical section is scoped to the loop body
> instead of an externally managed rcu_read_lock()/rcu_read_unlock()
> pair.
>
> Signed-off-by: Ye Liu <liuye@xxxxxxxxxx>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Some nits below, with those addressed, LGTM so:

Reviewed-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>

> ---
> Changes in v2:
>   - Rename macros from *_rcu to *_rculock (Steven Rostedt, Thomas Gleixner)
>   - Improve comment on for_each_process_thread_rculock() double loop
>   - Change prefix from mm: to sched: (Michal Hocko)
>  include/linux/sched/signal.h | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
>
> diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
> index 584ae88b435e..110ea9f131f3 100644
> --- a/include/linux/sched/signal.h
> +++ b/include/linux/sched/signal.h
> @@ -2,6 +2,7 @@
>  #ifndef _LINUX_SCHED_SIGNAL_H
>  #define _LINUX_SCHED_SIGNAL_H
>
> +#include <linux/cleanup.h>
>  #include <linux/rculist.h>
>  #include <linux/signal.h>
>  #include <linux/sched.h>
> @@ -663,6 +664,30 @@ extern bool current_is_single_threaded(void);
>  #define for_each_process_thread(p, t)	\
>  	for_each_process(p) for_each_thread(p, t)
>
> +/*
> + * Variants of for_each_process(), for_each_thread() and
> + * for_each_process_thread() that automatically acquire and release the
> + * RCU read lock via scoped_guard(rcu).  The lock is held for the
> + * duration of the loop and released on exit, including via break,
> + * goto, or return.
> + */
> +#define for_each_process_rculock(p) \
> +	scoped_guard(rcu) \
> +	for (p = &init_task ; (p = next_task(p)) != &init_task ; )

NIT: Can you indent the for loop by a tab? Since the for is in
scoped_guard() scope and that's not made clear.

> +
> +#define for_each_thread_rculock(p, t) \
> +	scoped_guard(rcu) \
> +	__for_each_thread((p)->signal, t)

Same comment as above.

> +
> +/*
> + * Double loop: 'break' only exits the inner for_each_thread() loop.
> + * Use 'goto' to exit both loops; the RCU read lock is released
> + * automatically when the scoped_guard scope is left.
> + */
> +#define for_each_process_thread_rculock(p, t) \
> +	scoped_guard(rcu) \
> +	for_each_process(p) for_each_thread(p, t)

Yeah this is even worse for clarity :) I know the existing defines do itg
this way but it's kinda confusing.

I know it's a bit fugly, but it's MUCH clearer as:

	#define for_each_process_thread_rculock(p, t) \
		scoped_guard(rcu) \
			for_each_process(p) \
				for_each_thread(p, t)

> +
>  typedef int (*proc_visitor)(struct task_struct *p, void *data);
>  void walk_process_tree(struct task_struct *top, proc_visitor, void *);
>
> --
> 2.25.1
>

--
Cheers, Lorenzo