get_task_mm: check PF_KTHREAD lockless

[PATCH] get_task_mm: check PF_KTHREAD lockless

Posted by Oleg Nesterov 1 year, 5 months ago

Nowadays PF_KTHREAD is sticky and it was never protected by ->alloc_lock.
Move the PF_KTHREAD check outside of task_lock() section to make this code
more understandable.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/fork.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 4c361d2bdc12..d3dc623a1247 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1536,14 +1536,13 @@ struct mm_struct *get_task_mm(struct task_struct *task)
 {
 	struct mm_struct *mm;
 
+	if (task->flags & PF_KTHREAD)
+		return NULL;
+
 	task_lock(task);
 	mm = task->mm;
-	if (mm) {
-		if (task->flags & PF_KTHREAD)
-			mm = NULL;
-		else
-			mmget(mm);
-	}
+	if (mm)
+		mmget(mm);
 	task_unlock(task);
 	return mm;
 }
-- 
2.25.1.362.g51ebf55

Re: [PATCH] get_task_mm: check PF_KTHREAD lockless

Posted by Michal Hocko 1 year, 5 months ago

On Wed 26-06-24 21:10:17, Oleg Nesterov wrote:
> Nowadays PF_KTHREAD is sticky and it was never protected by ->alloc_lock.
> Move the PF_KTHREAD check outside of task_lock() section to make this code
> more understandable.
> 
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

Acked-by: Michal Hocko <mhocko@suse.com>
Thanks!

> ---
>  kernel/fork.c | 11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 4c361d2bdc12..d3dc623a1247 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1536,14 +1536,13 @@ struct mm_struct *get_task_mm(struct task_struct *task)
>  {
>  	struct mm_struct *mm;
>  
> +	if (task->flags & PF_KTHREAD)
> +		return NULL;
> +
>  	task_lock(task);
>  	mm = task->mm;
> -	if (mm) {
> -		if (task->flags & PF_KTHREAD)
> -			mm = NULL;
> -		else
> -			mmget(mm);
> -	}
> +	if (mm)
> +		mmget(mm);
>  	task_unlock(task);
>  	return mm;
>  }
> -- 
> 2.25.1.362.g51ebf55
> 

-- 
Michal Hocko
SUSE Labs