signal: Don't disable preemption in ptrace_stop() on PREEMPT_RT

On PREEMPT_RT keeping preemption disabled during the invocation of
cgroup_enter_frozen() is a problem because the function acquires
css_set_lock which is a sleeping lock on PREEMPT_RT and must not be
acquired with disabled preemption.

The preempt-disabled section is only for performance optimisation reasons
and can be avoided.

Extend the comment and don't disable preemption before scheduling on
PREEMPT_RT.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20230803100932.325870-3-bigeasy@linutronix.de

kernel/signal.c

@@ -2345,10 +2345,21 @@ static int ptrace_stop(int exit_code, int why, unsigned long message,
 	 * will be no preemption between unlock and schedule() and so
 	 * improving the performance since the ptracer will observe that
 	 * the tracee is scheduled out once it gets on the CPU.
+	 *
+	 * On PREEMPT_RT locking tasklist_lock does not disable preemption.
+	 * Therefore the task can be preempted after do_notify_parent_cldstop()
+	 * before unlocking tasklist_lock so there is no benefit in doing this.
+	 *
+	 * In fact disabling preemption is harmful on PREEMPT_RT because
+	 * the spinlock_t in cgroup_enter_frozen() must not be acquired
+	 * with preemption disabled due to the 'sleeping' spinlock
+	 * substitution of RT.
 	 */
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
 		preempt_disable();
 	read_unlock(&tasklist_lock);
 	cgroup_enter_frozen();
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
 		preempt_enable_no_resched();
 	schedule();
 	cgroup_leave_frozen(true);