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: 76f969e ("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]>

Author: oleg-nesterov

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 {