[PATCH] cgroup: Remove redundant rcu_read_lock() in spin_lock_irq() section

[PATCH] cgroup: Remove redundant rcu_read_lock() in spin_lock_irq() section

[lirongqing]

From: Li RongQing <lirongqing@baidu.com>

Since spin_lock_irq() already disables preemption and task_css_set()
is protected by css_set_lock, the rcu_read_lock() calls are unnecessary
within the critical section. Remove them to simplify the code.

Signed-off-by: Li RongQing <lirongqing@baidu.com>
---
 kernel/cgroup/cgroup.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 312c6a8..db9e00a 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -2944,14 +2944,12 @@ int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
 
 	/* look up all src csets */
 	spin_lock_irq(&css_set_lock);
-	rcu_read_lock();
 	task = leader;
 	do {
 		cgroup_migrate_add_src(task_css_set(task), dst_cgrp, &mgctx);
 		if (!threadgroup)
 			break;
 	} while_each_thread(leader, task);
-	rcu_read_unlock();
 	spin_unlock_irq(&css_set_lock);
 
 	/* prepare dst csets and commit */
-- 
2.9.4

Re: [PATCH] cgroup: Remove redundant rcu_read_lock() in spin_lock_irq() section

[Chen Ridong]

On 2025/8/15 17:14, lirongqing wrote:
> From: Li RongQing <lirongqing@baidu.com>
> 
> Since spin_lock_irq() already disables preemption and task_css_set()
> is protected by css_set_lock, the rcu_read_lock() calls are unnecessary
> within the critical section. Remove them to simplify the code.
> 
> Signed-off-by: Li RongQing <lirongqing@baidu.com>
> ---
>  kernel/cgroup/cgroup.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index 312c6a8..db9e00a 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -2944,14 +2944,12 @@ int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
>  
>  	/* look up all src csets */
>  	spin_lock_irq(&css_set_lock);
> -	rcu_read_lock();
>  	task = leader;
>  	do {
>  		cgroup_migrate_add_src(task_css_set(task), dst_cgrp, &mgctx);
>  		if (!threadgroup)
>  			break;
>  	} while_each_thread(leader, task);
> -	rcu_read_unlock();
>  	spin_unlock_irq(&css_set_lock);
>  
>  	/* prepare dst csets and commit */

LGTM

-- 
Best regards,
Ridong

Re: [PATCH] cgroup: Remove redundant rcu_read_lock() in spin_lock_irq() section

[Michal Koutný]

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Hello RongQing.

On Fri, Aug 15, 2025 at 05:14:30PM +0800, lirongqing <lirongqing@baidu.com> wrote:
> From: Li RongQing <lirongqing@baidu.com>
> 
> Since spin_lock_irq() already disables preemption and task_css_set()
> is protected by css_set_lock, the rcu_read_lock() calls are unnecessary
> within the critical section. Remove them to simplify the code.
> 
> Signed-off-by: Li RongQing <lirongqing@baidu.com>

So there is some inconsistency betwen cgroup_migrate() and
cgroup_attach_task() (see also 674b745e22b3c ("cgroup: remove
rcu_read_lock()/rcu_read_unlock() in critical section of
spin_lock_irq()")) -- that'd warrant unification. Have you spotted other
instances of this?

The RCU lock is there not only because of task_css_set() but also for
while_each_thread(). I'd slightly prefer honoring the advice from Paul
[1] and keep a redundant rcu_read_lock() -- for more robustness to
reworks, I'm not convinced this simplification has othe visible
benefits.

Thanks,
Michal

[1] https://lore.kernel.org/all/20220107213612.GQ4202@paulmck-ThinkPad-P17-Gen-1/

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