[PATCH v4 2/9] ring-buffer: Hold cpu_buffer::lock when resizing a subbuf

[Vincent Donnefort <vdonnefort@google.com>]

Because, ring_buffer_subbuf_order_set() can clear cpu_buffer->free_page,
hold cpu_buffer->lock to prevent races with
ring_buffer_alloc_read_page() and ring_buffer_free_read_page().

Fixes: 8e7b58c27b3c ("ring-buffer: Just update the subbuffers when changing their allocation order")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index a3d28b2e2c94..ec4f5a0c93e8 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -7431,8 +7431,10 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
 		cpu_buffer->nr_pages = cpu_buffer->nr_pages_to_update;
 		cpu_buffer->nr_pages_to_update = 0;
 
+		arch_spin_lock(&cpu_buffer->lock);
 		old_free_data_page = cpu_buffer->free_page;
 		cpu_buffer->free_page = NULL;
+		arch_spin_unlock(&cpu_buffer->lock);
 
 		rb_head_page_activate(cpu_buffer);
 
-- 
2.55.0.691.gc56d675ccc-goog