linux-kernel - Re: [PATCH v3] Fixing directly deferencing a _

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Date:   Fri, 27 Oct 2023 02:50:12 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Abhinav Singh <singhabhinav9051571833@...il.com>
Cc:     akpm@...ux-foundation.org, brauner@...nel.org, surenb@...gle.com,
        michael.christie@...cle.com, mathieu.desnoyers@...icios.com,
        mjguzik@...il.com, npiggin@...il.com, shakeelb@...gle.com,
        peterz@...radead.org, linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [PATCH v3] Fixing directly deferencing a __rcu pointer warning

On Fri, Oct 27, 2023 at 12:07:13PM +0530, Abhinav Singh wrote:
> This patch fixes the warning about directly dereferencing a pointer
> tagged with __rcu annotation.
> 
> Dereferencing the pointers tagged with __rcu directly should
> always be avoided according to the docs. There is a rcu helper
> functions

function

> rcu_dereference(...) to use when dereferencing a __rcu
> pointer.


... inside rcu read side critical sections.

> This functions

function

> returns the non __rcu tagged pointer which
> can be dereferenced just like a normal pointers.

pointer

> 
>

Extra empty line here.
 

Did you test this with lockdep on or did you just build it?
Include info on how the patch was tested pls.

> Signed-off-by: Abhinav Singh <singhabhinav9051571833@...il.com>

> ---

Changelog?


>  kernel/fork.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 10917c3e1f03..e78649974669 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2369,7 +2369,7 @@ __latent_entropy struct task_struct *copy_process(
>  
>  	retval = -EAGAIN;
>  	if (is_rlimit_overlimit(task_ucounts(p), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC))) {
> -		if (p->real_cred->user != INIT_USER &&
> +		if (rcu_dereference(p->real_cred)->user != INIT_USER &&
>  		    !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
>  			goto bad_fork_cleanup_count;
>  	}
> @@ -2690,9 +2690,9 @@ __latent_entropy struct task_struct *copy_process(
>  			 * tasklist_lock with adding child to the process tree
>  			 * for propagate_has_child_subreaper optimization.
>  			 */
> -			p->signal->has_child_subreaper = p->real_parent->signal->has_child_subreaper ||
> -							 p->real_parent->signal->is_child_subreaper;
> -			list_add_tail(&p->sibling, &p->real_parent->children);
> +			p->signal->has_child_subreaper = rcu_dereference(p->real_parent)->signal->has_child_subreaper ||
> +							rcu_dereference(p->real_parent)->signal->is_child_subreaper;
> +			list_add_tail(&p->sibling, &rcu_dereference(p->real_parent)->children);
>  			list_add_tail_rcu(&p->tasks, &init_task.tasks);
>  			attach_pid(p, PIDTYPE_TGID);
>  			attach_pid(p, PIDTYPE_PGID);

It looks like you are calling rcu_dereference outside of
read side critical section and that does not look right to me.
Test with lockdep on.

> -- 
> 2.39.2