linux-kernel - [PATCH] get_task_mm: check PF

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Message-ID: <20240626191017.GA20031@redhat.com>
Date: Wed, 26 Jun 2024 21:10:17 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: "Eric W. Biederman" <ebiederm@...ssion.com>,
	Michal Hocko <mhocko@...e.com>, linux-kernel@...r.kernel.org
Subject: [PATCH] get_task_mm: check PF_KTHREAD lockless

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@...hat.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