ring-buffer: Check for empty ring-buffer with rb_num_of_entries()

Currently there are two ways of identifying an empty ring-buffer. One
relying on the current status of the commit / reader page
(rb_per_cpu_empty()) and the other on the write and read counters
(rb_num_of_entries() used in rb_get_reader_page()).

with rb_num_of_entries(). This intends to ease later
introduction of ring-buffer writers which are out of the kernel control
and with whom, the only information available is through the meta-page
counters.

Link: https://lore.kernel.org/20250108114536.627715-2-vdonnefort@google.com
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 60210fb5b211039aabbf74033c38f9bd385c03bd..479bbbcbf61b187adc1dbbe490c420d9b95a95ea 100644 (file)
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -4682,40 +4682,22 @@ int ring_buffer_write(struct trace_buffer *buffer,
 }
 EXPORT_SYMBOL_GPL(ring_buffer_write);
 
-static bool rb_per_cpu_empty(struct ring_buffer_per_cpu *cpu_buffer)
+/*
+ * The total entries in the ring buffer is the running counter
+ * of entries entered into the ring buffer, minus the sum of
+ * the entries read from the ring buffer and the number of
+ * entries that were overwritten.
+ */
+static inline unsigned long
+rb_num_of_entries(struct ring_buffer_per_cpu *cpu_buffer)
 {
-       struct buffer_page *reader = cpu_buffer->reader_page;
-       struct buffer_page *head = rb_set_head_page(cpu_buffer);
-       struct buffer_page *commit = cpu_buffer->commit_page;
-
-       /* In case of error, head will be NULL */
-       if (unlikely(!head))
-               return true;
-
-       /* Reader should exhaust content in reader page */
-       if (reader->read != rb_page_size(reader))
-               return false;
-
-       /*
-        * If writers are committing on the reader page, knowing all
-        * committed content has been read, the ring buffer is empty.
-        */
-       if (commit == reader)
-               return true;
-
-       /*
-        * If writers are committing on a page other than reader page
-        * and head page, there should always be content to read.
-        */
-       if (commit != head)
-               return false;
+       return local_read(&cpu_buffer->entries) -
+               (local_read(&cpu_buffer->overrun) + cpu_buffer->read);
+}
 
-       /*
-        * Writers are committing on the head page, we just need
-        * to care about there're committed data, and the reader will
-        * swap reader page with head page when it is to read data.
-        */
-       return rb_page_commit(commit) == 0;
+static bool rb_per_cpu_empty(struct ring_buffer_per_cpu *cpu_buffer)
+{
+       return !rb_num_of_entries(cpu_buffer);
 }
 
 /**
@@ -4861,19 +4843,6 @@ void ring_buffer_record_enable_cpu(struct trace_buffer *buffer, int cpu)
 }
 EXPORT_SYMBOL_GPL(ring_buffer_record_enable_cpu);
 
-/*
- * The total entries in the ring buffer is the running counter
- * of entries entered into the ring buffer, minus the sum of
- * the entries read from the ring buffer and the number of
- * entries that were overwritten.
- */
-static inline unsigned long
-rb_num_of_entries(struct ring_buffer_per_cpu *cpu_buffer)
-{
-       return local_read(&cpu_buffer->entries) -
-               (local_read(&cpu_buffer->overrun) + cpu_buffer->read);
-}
-
 /**
  * ring_buffer_oldest_event_ts - get the oldest event timestamp from the buffer
  * @buffer: The ring buffer