Commit 0568c6eb authored by Vincent Donnefort's avatar Vincent Donnefort Committed by Steven Rostedt (Google)
Browse files

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: default avatarVincent Donnefort <vdonnefort@google.com>
Signed-off-by: default avatarSteven Rostedt (Google) <rostedt@goodmis.org>
parent 5bc55a33

kernel/trace/ring_buffer.c

+14 −45
Original line number Diff line number Diff line
@@ -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)

{

	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.

 * 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.

 */

	if (commit != head)

		return false;

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);

}



	/*

	 * 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