Re: [PATCH v2] ring-buffer: Handle a reader taking the new head in rb_handle_head_page()

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 5 Nov 2026 15:57:29 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> We may need to add memory barriers here, but for now, I think this could
> fix it. As it should prevent the search from ever starting in between the
> two heads.

Yeah, I think this will do it. Please test, I'll go write up a formal patch:

-- Steve

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index c64e001c3fc0..0d37f8b529d5 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -1494,7 +1494,13 @@ rb_set_head_page(struct ring_buffer_per_cpu *cpu_buffer)
 	if (RB_WARN_ON(cpu_buffer, rb_list_head(list->prev->next) != list))
 		return NULL;
 
-	page = head = cpu_buffer->head_page;
+	/*
+	 * Make sure that the start of the search of the head_page does not
+	 * start between the writer adding the UPDATE and the new HEAD that it
+	 * moves forward.
+	 */
+	/* Sync with the writer moving the HEAD in rb_handle_head_page() */
+	page = head = smp_load_acquire(&cpu_buffer->head_page);
 	/*
 	 * It is possible that the writer moves the header behind
 	 * where we started, and we miss in one loop.
@@ -3721,6 +3727,15 @@ rb_handle_head_page(struct ring_buffer_per_cpu *cpu_buffer,
 		local_sub(rb_page_commit(next_page), &cpu_buffer->entries_bytes);
 		local_inc(&cpu_buffer->pages_lost);
 
+		/*
+		 * To prevent a stale head_page from being between
+		 * the UPDATE and new HEAD pages, set it here to have
+		 * the reader check always find the update before
+		 * finding the HEAD page.
+		 */
+		/* Sync with the reader setting the head page */
+		smp_store_release(&cpu_buffer->head_page, next_page);
+
 		if (cpu_buffer->ring_meta)
 			rb_update_meta_head(cpu_buffer, next_page);
 		/*
-- 
2.53.0