Ring buffer fixes for 6.12:

- Revert: "ring-buffer: Do not have boot mapped buffers hook to CPU hotplug"
 
   A crash that happened on cpu hotplug was actually caused by the incorrect
   ref counting that was fixed by commit 2cf9733891 ("ring-buffer: Fix
   refcount setting of boot mapped buffers"). The removal of calling cpu
   hotplug callbacks on memory mapped buffers was not an issue even though
   the tests at the time pointed toward it. But in fact, there's a check in
   that code that tests to see if the buffers are already allocated or not,
   and will not allocate them again if they are. Not calling the cpu hotplug
   callbacks ended up not initializing the non boot CPU buffers.
 
   Simply remove that change.
 
 - Clear all CPU buffers when starting tracing in a boot mapped buffer
 
   To properly process events from a previous boot, the address space needs to
   be accounted for due to KASLR and the events in the buffer are updated
   accordingly when read. This also requires that when the buffer has tracing
   enabled again in the current boot that the buffers are reset so that events
   from the previous boot do not interact with the events of the current boot
   and cause confusing due to not having the proper meta data.
 
   It was found that if a CPU is taken offline, that its per CPU buffer is not
   reset when tracing starts. This allows for events to be from both the
   previous boot and the current boot to be in the buffer at the same time.
   Clear all CPU buffers when tracing is started in a boot mapped buffer.
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Merge tag 'trace-ringbuffer-v6.12-rc7-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace

Pull ring buffer fixes from Steven Rostedt:

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

   A crash that happened on cpu hotplug was actually caused by the
   incorrect ref counting that was fixed by commit 2cf9733891
   ("ring-buffer: Fix refcount setting of boot mapped buffers"). The
   removal of calling cpu hotplug callbacks on memory mapped buffers was
   not an issue even though the tests at the time pointed toward it. But
   in fact, there's a check in that code that tests to see if the
   buffers are already allocated or not, and will not allocate them
   again if they are. Not calling the cpu hotplug callbacks ended up not
   initializing the non boot CPU buffers.

   Simply remove that change.

 - Clear all CPU buffers when starting tracing in a boot mapped buffer

   To properly process events from a previous boot, the address space
   needs to be accounted for due to KASLR and the events in the buffer
   are updated accordingly when read. This also requires that when the
   buffer has tracing enabled again in the current boot that the buffers
   are reset so that events from the previous boot do not interact with
   the events of the current boot and cause confusing due to not having
   the proper meta data.

   It was found that if a CPU is taken offline, that its per CPU buffer
   is not reset when tracing starts. This allows for events to be from
   both the previous boot and the current boot to be in the buffer at
   the same time. Clear all CPU buffers when tracing is started in a
   boot mapped buffer.

* tag 'trace-ringbuffer-v6.12-rc7-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace:
  tracing/ring-buffer: Clear all memory mapped CPU ring buffers on first recording
  Revert: "ring-buffer: Do not have boot mapped buffers hook to CPU hotplug"

kernel/trace/ring_buffer.c

@@ -2337,12 +2337,9 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
 	if (!buffer->buffers[cpu])
 		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);
 

kernel/trace/trace.c

@@ -2386,6 +2386,25 @@ void tracing_reset_online_cpus(struct array_buffer *buf)
 	ring_buffer_record_enable(buffer);
 }
 
+static void tracing_reset_all_cpus(struct array_buffer *buf)
+{
+	struct trace_buffer *buffer = buf->buffer;
+
+	if (!buffer)
+		return;
+
+	ring_buffer_record_disable(buffer);
+
+	/* Make sure all commits have finished */
+	synchronize_rcu();
+
+	buf->time_start = buffer_ftrace_now(buf, buf->cpu);
+
+	ring_buffer_reset(buffer);
+
+	ring_buffer_record_enable(buffer);
+}
+
 /* Must have trace_types_lock held */
 void tracing_reset_all_online_cpus_unlocked(void)
 {
@@ -6145,8 +6164,13 @@ static void update_last_data(struct trace_array *tr)
 	if (!tr->text_delta && !tr->data_delta)
 		return;
 
-	/* Clear old data */
+	/*
-	tracing_reset_online_cpus(&tr->array_buffer);
+	 * Need to clear all CPU buffers as there cannot be events
+	 * from the previous boot mixed with events with this boot
+	 * as that will cause a confusing trace. Need to clear all
+	 * CPU buffers, even for those that may currently be offline.
+	 */
+	tracing_reset_all_cpus(&tr->array_buffer);
 
 	/* Using current data now */
 	tr->text_delta = 0;