linux-kernel - Re: [PATCH 1/1] release_task: kill unnecessary rcu_read_lock() around dec_rlimit

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Message-ID: <aP5B2tKhFAcdKhlO@example.org>
Date: Sun, 26 Oct 2025 16:44:26 +0100
From: Alexey Gladkov <legion@...nel.org>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	Mateusz Guzik <mjguzik@...il.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] release_task: kill unnecessary rcu_read_lock()
 around dec_rlimit_ucounts()

On Sun, Oct 26, 2025 at 03:31:40PM +0100, Oleg Nesterov wrote:
> rcu_read_lock() was added to shut RCU-lockdep up when this code used
> __task_cred()->rcu_dereference(), but after the commit 21d1c5e386bc
> ("Reimplement RLIMIT_NPROC on top of ucounts") it is no longer needed:
> task_ucounts()->task_cred_xxx() takes rcu_read_lock() itself.

Yes, it makes sense.

Acked-by: Alexey Gladkov <legion@...nel.org>

> NOTE: task_ucounts() returns the pointer to another rcu-protected data,
> struct ucounts. So it should either be used when task->real_cred and thus
> task->real_cred->ucounts is stable (release_task, copy_process, copy_creds),
> or it should be called under rcu_read_lock(). In both cases it is pointless
> to take rcu_read_lock() to read the cred->ucounts pointer.
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> ---
>  kernel/exit.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 9f74e8f1c431..f041f0c05ebb 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -251,10 +251,8 @@ void release_task(struct task_struct *p)
>  	memset(&post, 0, sizeof(post));
>  
>  	/* don't need to get the RCU readlock here - the process is dead and
> -	 * can't be modifying its own credentials. But shut RCU-lockdep up */
> -	rcu_read_lock();
> +	 * can't be modifying its own credentials. */
>  	dec_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
> -	rcu_read_unlock();
>  
>  	pidfs_exit(p);
>  	cgroup_release(p);
> -- 
> 2.25.1.362.g51ebf55
> 
> 

-- 
Rgrds, legion