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 <[email protected]>
Fixes: 76f969e8948d ("cgroup: cgroup v2 freezer")
Cc: [email protected] # v5.2+
Signed-off-by: Oleg Nesterov <[email protected]>
Acked-by: Roman Gushchin <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>

Change-Id: Ic53e0f2d6624b0bb90817b0c57060fb7db971348
(cherry picked from commit 937c6b27c73e02cd4114f95f5c37ba2c29fadba1)
Bug: 154548692
Signed-off-by: Marco Ballesio <[email protected]>

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 {