linux-kernel - [PATCH v3 3/6] exit: postpone tty_kref_put() until after tasklist

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Message-ID: <20250201163106.28912-4-mjguzik@gmail.com>
Date: Sat,  1 Feb 2025 17:31:03 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: ebiederm@...ssion.com,
	oleg@...hat.com
Cc: brauner@...nel.org,
	akpm@...ux-foundation.org,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Mateusz Guzik <mjguzik@...il.com>
Subject: [PATCH v3 3/6] exit: postpone tty_kref_put() until after tasklist_lock is dropped

Instead of smuggling the tty pointer directly, use a struct so that more
things can be added later.

Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
---
 kernel/exit.c | 22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 257dd8ed45ea..d2c74f93f7d2 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -122,6 +122,13 @@ static __init int kernel_exit_sysfs_init(void)
 late_initcall(kernel_exit_sysfs_init);
 #endif
 
+/*
+ * For things release_task() would like to do *after* tasklist_lock is released.
+ */
+struct release_task_post {
+	struct tty_struct *tty;
+};
+
 static void __unhash_process(struct task_struct *p, bool group_dead)
 {
 	nr_threads--;
@@ -141,12 +148,11 @@ static void __unhash_process(struct task_struct *p, bool group_dead)
 /*
  * This function expects the tasklist_lock write-locked.
  */
-static void __exit_signal(struct task_struct *tsk)
+static void __exit_signal(struct release_task_post *post, struct task_struct *tsk)
 {
 	struct signal_struct *sig = tsk->signal;
 	bool group_dead = thread_group_leader(tsk);
 	struct sighand_struct *sighand;
-	struct tty_struct *tty;
 	u64 utime, stime;
 
 	sighand = rcu_dereference_check(tsk->sighand,
@@ -160,7 +166,7 @@ static void __exit_signal(struct task_struct *tsk)
 #endif
 
 	if (group_dead) {
-		tty = sig->tty;
+		post->tty = sig->tty;
 		sig->tty = NULL;
 	} else {
 		/*
@@ -207,10 +213,8 @@ static void __exit_signal(struct task_struct *tsk)
 
 	__cleanup_sighand(sighand);
 	clear_tsk_thread_flag(tsk, TIF_SIGPENDING);
-	if (group_dead) {
+	if (group_dead)
 		flush_sigqueue(&sig->shared_pending);
-		tty_kref_put(tty);
-	}
 }
 
 static void delayed_put_task_struct(struct rcu_head *rhp)
@@ -236,10 +240,13 @@ void __weak release_thread(struct task_struct *dead_task)
 
 void release_task(struct task_struct *p)
 {
+	struct release_task_post post;
 	struct task_struct *leader;
 	struct pid *thread_pid;
 	int zap_leader;
 repeat:
+	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();
@@ -252,7 +259,7 @@ void release_task(struct task_struct *p)
 
 	write_lock_irq(&tasklist_lock);
 	ptrace_release_task(p);
-	__exit_signal(p);
+	__exit_signal(&post, p);
 
 	/*
 	 * If we are the last non-leader member of the thread
@@ -280,6 +287,7 @@ void release_task(struct task_struct *p)
 			      sizeof(unsigned long long));
 	release_thread(p);
 	put_task_struct_rcu_user(p);
+	tty_kref_put(post.tty);
 
 	p = leader;
 	if (unlikely(zap_leader))
-- 
2.43.0