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Date:   Wed, 9 Oct 2019 17:02:30 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Tejun Heo <tj@kernel.org>
Cc:     linux-kernel@vger.kernel.org, Roman Gushchin <guro@fb.com>,
        Richard Purdie <richard.purdie@linuxfoundation.org>,
        Bruce Ashfield <bruce.ashfield@gmail.com>
Subject: [PATCH] cgroup: freezer: call cgroup_enter_frozen() with preemption
 disabled in ptrace_stop()
Message-ID: <20191009150230.GB12511@redhat.com>
References: <CADkTA4PBT374CY+UNb85WjQEaNCDodMZu=MgpG8aMYbAu2eOGA@mail.gmail.com>
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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>
---
 kernel/signal.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 534fec2..f8eed86 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2205,8 +2205,8 @@ static void ptrace_stop(int exit_code, int why, int clear_code, kernel_siginfo_t
 		 */
 		preempt_disable();
 		read_unlock(&tasklist_lock);
-		preempt_enable_no_resched();
 		cgroup_enter_frozen();
+		preempt_enable_no_resched();
 		freezable_schedule();
 		cgroup_leave_frozen(true);
 	} else {
-- 
2.5.0