Write to buffer_size_kb can permanently fail due to cpu_online_mask changed
between two for_each_online_buffer_cpu loops.
The number of increasing and decreasing on cpu_buffer->resize_disable
may be inconsistent, leading that the resize_disabled in some CPUs
becoming none zero after ring_buffer_reset_online_cpus return.

This issue can be reproduced by "echo 0 > trace" and hotplug cpu at the
same time. After reproducing success, we can find out buffer_size_kb
will not be functional anymore.

This patch uses cpus_read_lock() to prevent cpu_online_mask being changed
between two different "for_each_online_buffer_cpu".

Changes in v2:
  Use cpus_read_lock() instead of copying cpu_online_mask at the entry of
  function

Link:
  https://lore.kernel.org/lkml/20230408052226.25268-1-Tze-nan.Wu@mediatek.com/ (local)

Cc: stable@vger.kernel.org
Signed-off-by: Tze-nan Wu <redacted>
---
 kernel/trace/ring_buffer.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 76a2d91eecad..44d833252fb0 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c

@@ -5357,6 +5357,7 @@ void ring_buffer_reset_online_cpus(struct trace_buffer *buffer)
 
 	/* prevent another thread from changing buffer sizes */
 	mutex_lock(&buffer->mutex);
+	cpus_read_lock();
 
 	for_each_online_buffer_cpu(buffer, cpu) {
 		cpu_buffer = buffer->buffers[cpu];

@@ -5377,6 +5378,7 @@ void ring_buffer_reset_online_cpus(struct trace_buffer *buffer)
 		atomic_dec(&cpu_buffer->resize_disabled);
 	}
 
+	cpus_read_unlock();
 	mutex_unlock(&buffer->mutex);
 }
 

-- 
2.18.0