[PATCH v1 3/4] ring-buffer: Order remote page commit with data and timestamp

[Vincent Donnefort <vdonnefort@google.com>]

__rb_get_reader_page_from_remote() uses page::commit to check if more
data is available for consumption. However, it needs to make sure that
data and page::time_stamp are not stale. Enforce page::commit order with
page::data and page::time_stamp, and defer updating read_stamp on page
swap until rb_page_size() confirms committed data is present.

Cc: stable@vger.kernel.org
Fixes: 2e67fabd8b77 ("ring-buffer: Introduce ring-buffer remotes")
Fixes: 34e5b958bdad ("tracing: Introduce simple_ring_buffer")
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
---
 kernel/trace/ring_buffer.c        | 23 ++++++++++++++++-------
 kernel/trace/simple_ring_buffer.c |  5 +++++
 2 files changed, 21 insertions(+), 7 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index f70010e2ede7..6016d16d1566 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -5832,11 +5832,8 @@ __rb_get_reader_page_from_remote(struct ring_buffer_per_cpu *cpu_buffer)
 	smp_rmb();
 
 	/* More to read on the reader page */
-	if (cpu_buffer->reader_page->read < rb_page_size(cpu_buffer->reader_page)) {
-		if (!cpu_buffer->reader_page->read)
-			cpu_buffer->read_stamp = cpu_buffer->reader_page->page->time_stamp;
-		return cpu_buffer->reader_page;
-	}
+	if (cpu_buffer->reader_page->read < rb_page_size(cpu_buffer->reader_page))
+		goto out;
 
 	if (!rb_num_of_entries(cpu_buffer))
 		return NULL;
@@ -5883,10 +5880,22 @@ __rb_get_reader_page_from_remote(struct ring_buffer_per_cpu *cpu_buffer)
 	cpu_buffer->reader_page = new_reader;
 	cpu_buffer->reader_page->read = 0;
 	cpu_buffer->pages = &new_head->list;
-	cpu_buffer->read_stamp = new_reader->page->time_stamp;
 	cpu_buffer->lost_events = cpu_buffer->meta_page->reader.lost_events;
 
-	return rb_page_size(cpu_buffer->reader_page) ? cpu_buffer->reader_page : NULL;
+	if (!rb_page_size(cpu_buffer->reader_page))
+		return NULL;
+
+out:
+	/*
+	 * Order page::commit read before page::data and page::time_stamp, most
+	 * likely paired with simple_ring_buffer_commit().
+	 */
+	smp_rmb();
+
+	if (!cpu_buffer->reader_page->read)
+		cpu_buffer->read_stamp = cpu_buffer->reader_page->page->time_stamp;
+
+	return cpu_buffer->reader_page;
 }
 
 static struct buffer_page *
diff --git a/kernel/trace/simple_ring_buffer.c b/kernel/trace/simple_ring_buffer.c
index 5f14ba152fba..2dddaef638dc 100644
--- a/kernel/trace/simple_ring_buffer.c
+++ b/kernel/trace/simple_ring_buffer.c
@@ -307,6 +307,11 @@ EXPORT_SYMBOL_GPL(simple_ring_buffer_reserve);
  */
 void simple_ring_buffer_commit(struct simple_rb_per_cpu *cpu_buffer)
 {
+	/*
+	 * Order page::data and page::time_stamp before page::commit. Paired
+	 * with __rb_get_reader_page_from_remote().
+	 */
+	smp_wmb();
 	local_set(&cpu_buffer->tail_page->page->commit,
 		  cpu_buffer->tail_page->write);
 
-- 
2.56.0.611.g1030f0b940-goog