ring-buffer: Do not have boot mapped buffers hook to CPU hotplug

The boot mapped ring buffer has its buffer mapped at a fixed location
found at boot up. It is not dynamic. It cannot grow or be expanded when
new CPUs come online.

Do not hook fixed memory mapped ring buffers to the CPU hotplug callback,
otherwise it can cause a crash when it tries to add the buffer to the
memory that is already fully occupied.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lore.kernel.org/20241008143242.25e20801@gandalf.local.home
Fixes: be68d63a ("ring-buffer: Add ring_buffer_alloc_range()")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

kernel/trace/ring_buffer.c

@@ -2337,9 +2337,12 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
 	if (!buffer->buffers[cpu])
 		goto fail_free_buffers;
 
-	ret = cpuhp_state_add_instance(CPUHP_TRACE_RB_PREPARE, &buffer->node);
-	if (ret < 0)
-		goto fail_free_buffers;
+	/* If already mapped, do not hook to CPU hotplug */
+	if (!start) {
+		ret = cpuhp_state_add_instance(CPUHP_TRACE_RB_PREPARE, &buffer->node);
+		if (ret < 0)
+			goto fail_free_buffers;
+	}
 
 	mutex_init(&buffer->mutex);