From caf7caca0b7883e3f4df0488e3f23ced5ade9ade Mon Sep 17 00:00:00 2001
From: Oleg Nesterov <oleg@redhat.com>
Date: Wed, 9 Oct 2019 17:02:30 +0200
Subject: [PATCH] UPSTREAM: cgroup: freezer: call cgroup_enter_frozen() with
 preemption disabled in ptrace_stop()

ptrace_stop() does preempt_enable_no_resched() to avoid the preemption,
but after that cgroup_enter_frozen() does spin_lock/unlock and this adds
another preemption point.

Reported-and-tested-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Fixes: 76f969e8948d ("cgroup: cgroup v2 freezer")
Cc: stable@vger.kernel.org # v5.2+
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Change-Id: Ic53e0f2d6624b0bb90817b0c57060fb7db971348
(cherry picked from commit 937c6b27c73e02cd4114f95f5c37ba2c29fadba1)
Bug: 154548692
Signed-off-by: Marco Ballesio <balejs@google.com>
---
 kernel/signal.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index bb2bf761f422..1c5140a75cee 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2131,8 +2131,8 @@ static void ptrace_stop(int exit_code, int why, int clear_code, siginfo_t *info)
 		 */
 		preempt_disable();
 		read_unlock(&tasklist_lock);
-		preempt_enable_no_resched();
 		cgroup_enter_frozen();
+		preempt_enable_no_resched();
 		freezable_schedule();
 		cgroup_leave_frozen(true);
 	} else {