linux-kernel - [PATCH v5 1/5] exit: perform add_device_randomness() without tasklist

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Message-ID: <20250205200929.406568-2-mjguzik@gmail.com>
Date: Wed,  5 Feb 2025 21:09:25 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: ebiederm@...ssion.com,
	oleg@...hat.com
Cc: brauner@...nel.org,
	akpm@...ux-foundation.org,
	Liam.Howlett@...cle.com,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Mateusz Guzik <mjguzik@...il.com>
Subject: [PATCH v5 1/5] exit: perform add_device_randomness() without tasklist_lock

Parallel calls to add_device_randomness() contend on their own.

The clone side aleady runs outside of tasklist_lock, which in turn means
any caller on the exit side extends the tasklist_lock hold time while
contending on the random-private lock.

Reviewed-by: Oleg Nesterov <oleg@...hat.com>
Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
---
 kernel/exit.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 3485e5fc499e..c79b41509cd3 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -174,9 +174,6 @@ static void __exit_signal(struct task_struct *tsk)
 			sig->curr_target = next_thread(tsk);
 	}
 
-	add_device_randomness((const void*) &tsk->se.sum_exec_runtime,
-			      sizeof(unsigned long long));
-
 	/*
 	 * Accumulate here the counters for all threads as they die. We could
 	 * skip the group leader because it is the last user of signal_struct,
@@ -278,6 +275,9 @@ void release_task(struct task_struct *p)
 	write_unlock_irq(&tasklist_lock);
 	proc_flush_pid(thread_pid);
 	put_pid(thread_pid);
+	add_device_randomness(&p->se.sum_exec_runtime,
+			      sizeof(p->se.sum_exec_runtime));
+	free_pids(post.pids);
 	release_thread(p);
 	put_task_struct_rcu_user(p);
 
-- 
2.43.0