kernel/trace/simple_ring_buffer.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Copyright (C) 2025 - Google LLC
 * Author: Vincent Donnefort <vdonnefort@google.com>
 */

#include <linux/atomic.h>
#include <linux/simple_ring_buffer.h>

#include <asm/barrier.h>
#include <asm/local.h>

enum simple_rb_link_type {
	SIMPLE_RB_LINK_NORMAL		= 0,
	SIMPLE_RB_LINK_HEAD		= 1,
	SIMPLE_RB_LINK_HEAD_MOVING
};

#define SIMPLE_RB_LINK_MASK ~(SIMPLE_RB_LINK_HEAD | SIMPLE_RB_LINK_HEAD_MOVING)

static void simple_bpage_set_head_link(struct simple_buffer_page *bpage)
{
	unsigned long link = (unsigned long)bpage->link.next;

	link &= SIMPLE_RB_LINK_MASK;
	link |= SIMPLE_RB_LINK_HEAD;

	/*
	 * Paired with simple_rb_find_head() to order access between the head
	 * link and overrun. It ensures we always report an up-to-date value
	 * after swapping the reader page.
	 */
	smp_store_release(&bpage->link.next, (struct list_head *)link);
}

static bool simple_bpage_unset_head_link(struct simple_buffer_page *bpage,
					 struct simple_buffer_page *dst,
					 enum simple_rb_link_type new_type)
{
	unsigned long *link = (unsigned long *)(&bpage->link.next);
	unsigned long old = (*link & SIMPLE_RB_LINK_MASK) | SIMPLE_RB_LINK_HEAD;
	unsigned long new = (unsigned long)(&dst->link) | new_type;

	return try_cmpxchg(link, &old, new);
}

static void simple_bpage_set_normal_link(struct simple_buffer_page *bpage)
{
	unsigned long link = (unsigned long)bpage->link.next;

	WRITE_ONCE(bpage->link.next, (struct list_head *)(link & SIMPLE_RB_LINK_MASK));
}

static struct simple_buffer_page *simple_bpage_from_link(struct list_head *link)
{
	unsigned long ptr = (unsigned long)link & SIMPLE_RB_LINK_MASK;

	return container_of((struct list_head *)ptr, struct simple_buffer_page, link);
}

static struct simple_buffer_page *simple_bpage_next_page(struct simple_buffer_page *bpage)
{
	return simple_bpage_from_link(bpage->link.next);
}

static void simple_bpage_reset(struct simple_buffer_page *bpage)
{
	bpage->write = 0;
	bpage->entries = 0;

	local_set(&bpage->page->commit, 0);
}

static void simple_bpage_init(struct simple_buffer_page *bpage, unsigned long page)
{
	INIT_LIST_HEAD(&bpage->link);
	bpage->page = (struct buffer_data_page *)page;

	simple_bpage_reset(bpage);
}

#define simple_rb_meta_inc(__meta, __inc)		\
	WRITE_ONCE((__meta), (__meta + __inc))

static bool simple_rb_loaded(struct simple_rb_per_cpu *cpu_buffer)
{
	return !!cpu_buffer->bpages;
}

static int simple_rb_find_head(struct simple_rb_per_cpu *cpu_buffer)
{
	int retry = cpu_buffer->nr_pages * 2;
	struct simple_buffer_page *head;

	head = cpu_buffer->head_page;

	while (retry--) {
		unsigned long link;

spin:
		/* See smp_store_release in simple_bpage_set_head_link() */
		link = (unsigned long)smp_load_acquire(&head->link.prev->next);

		switch (link & ~SIMPLE_RB_LINK_MASK) {
		/* Found the head */
		case SIMPLE_RB_LINK_HEAD:
			cpu_buffer->head_page = head;
			return 0;
		/* The writer caught the head, we can spin, that won't be long */
		case SIMPLE_RB_LINK_HEAD_MOVING:
			goto spin;
		}

		head = simple_bpage_next_page(head);
	}

	return -EBUSY;
}

/**
 * simple_ring_buffer_swap_reader_page - Swap ring-buffer head with the reader
 * @cpu_buffer: A simple_rb_per_cpu
 *
 * This function enables consuming reading. It ensures the current head page will not be overwritten
 * and can be safely read.
 *
 * Returns 0 on success, -ENODEV if @cpu_buffer was unloaded or -EBUSY if we failed to catch the
 * head page.
 */
int simple_ring_buffer_swap_reader_page(struct simple_rb_per_cpu *cpu_buffer)
{
	struct simple_buffer_page *last, *head, *reader;
	unsigned long overrun;
	int retry = 8;
	int ret;

	if (!simple_rb_loaded(cpu_buffer))
		return -ENODEV;

	reader = cpu_buffer->reader_page;

	do {
		/* Run after the writer to find the head */
		ret = simple_rb_find_head(cpu_buffer);
		if (ret)
			return ret;

		head = cpu_buffer->head_page;

		/* Connect the reader page around the header page */
		reader->link.next = head->link.next;
		reader->link.prev = head->link.prev;

		/* The last page before the head */
		last = simple_bpage_from_link(head->link.prev);

		/* The reader page points to the new header page */
		simple_bpage_set_head_link(reader);

		overrun = cpu_buffer->meta->overrun;
	} while (!simple_bpage_unset_head_link(last, reader, SIMPLE_RB_LINK_NORMAL) && retry--);

	if (!retry)
		return -EINVAL;

	cpu_buffer->head_page = simple_bpage_from_link(reader->link.next);
	cpu_buffer->head_page->link.prev = &reader->link;
	cpu_buffer->reader_page = head;
	cpu_buffer->meta->reader.lost_events = overrun - cpu_buffer->last_overrun;
	cpu_buffer->meta->reader.id = cpu_buffer->reader_page->id;
	cpu_buffer->last_overrun = overrun;

	return 0;
}
EXPORT_SYMBOL_GPL(simple_ring_buffer_swap_reader_page);

static struct simple_buffer_page *simple_rb_move_tail(struct simple_rb_per_cpu *cpu_buffer)
{
	struct simple_buffer_page *tail, *new_tail;

	tail = cpu_buffer->tail_page;
	new_tail = simple_bpage_next_page(tail);

	if (simple_bpage_unset_head_link(tail, new_tail, SIMPLE_RB_LINK_HEAD_MOVING)) {
		/*
		 * Oh no! we've caught the head. There is none anymore and
		 * swap_reader will spin until we set the new one. Overrun must
		 * be written first, to make sure we report the correct number
		 * of lost events.
		 */
		simple_rb_meta_inc(cpu_buffer->meta->overrun, new_tail->entries);
		simple_rb_meta_inc(cpu_buffer->meta->pages_lost, 1);

		simple_bpage_set_head_link(new_tail);
		simple_bpage_set_normal_link(tail);
	}

	simple_bpage_reset(new_tail);
	cpu_buffer->tail_page = new_tail;

	simple_rb_meta_inc(cpu_buffer->meta->pages_touched, 1);

	return new_tail;
}

static unsigned long rb_event_size(unsigned long length)
{
	struct ring_buffer_event *event;

	return length + RB_EVNT_HDR_SIZE + sizeof(event->array[0]);
}

static struct ring_buffer_event *
rb_event_add_ts_extend(struct ring_buffer_event *event, u64 delta)
{
	event->type_len = RINGBUF_TYPE_TIME_EXTEND;
	event->time_delta = delta & TS_MASK;
	event->array[0] = delta >> TS_SHIFT;

	return (struct ring_buffer_event *)((unsigned long)event + 8);
}

static struct ring_buffer_event *
simple_rb_reserve_next(struct simple_rb_per_cpu *cpu_buffer, unsigned long length, u64 timestamp)
{
	unsigned long ts_ext_size = 0, event_size = rb_event_size(length);
	struct simple_buffer_page *tail = cpu_buffer->tail_page;
	struct ring_buffer_event *event;
	u32 write, prev_write;
	u64 time_delta;

	time_delta = timestamp - cpu_buffer->write_stamp;

	if (test_time_stamp(time_delta))
		ts_ext_size = 8;

	prev_write = tail->write;
	write = prev_write + event_size + ts_ext_size;

	if (unlikely(write > (PAGE_SIZE - BUF_PAGE_HDR_SIZE)))
		tail = simple_rb_move_tail(cpu_buffer);

	if (!tail->entries) {
		tail->page->time_stamp = timestamp;
		time_delta = 0;
		ts_ext_size = 0;
		write = event_size;
		prev_write = 0;
	}

	tail->write = write;
	tail->entries++;

	cpu_buffer->write_stamp = timestamp;

	event = (struct ring_buffer_event *)(tail->page->data + prev_write);
	if (ts_ext_size) {
		event = rb_event_add_ts_extend(event, time_delta);
		time_delta = 0;
	}

	event->type_len = 0;
	event->time_delta = time_delta;
	event->array[0] = event_size - RB_EVNT_HDR_SIZE;

	return event;
}

/**
 * simple_ring_buffer_reserve - Reserve an entry in @cpu_buffer
 * @cpu_buffer:	A simple_rb_per_cpu
 * @length:	Size of the entry in bytes
 * @timestamp:	Timestamp of the entry
 *
 * Returns the address of the entry where to write data or NULL
 */
void *simple_ring_buffer_reserve(struct simple_rb_per_cpu *cpu_buffer, unsigned long length,
				 u64 timestamp)
{
	struct ring_buffer_event *rb_event;

	if (cmpxchg(&cpu_buffer->status, SIMPLE_RB_READY, SIMPLE_RB_WRITING) != SIMPLE_RB_READY)
		return NULL;

	rb_event = simple_rb_reserve_next(cpu_buffer, length, timestamp);

	return &rb_event->array[1];
}
EXPORT_SYMBOL_GPL(simple_ring_buffer_reserve);

/**
 * simple_ring_buffer_commit - Commit the entry reserved with simple_ring_buffer_reserve()
 * @cpu_buffer:	The simple_rb_per_cpu where the entry has been reserved
 */
void simple_ring_buffer_commit(struct simple_rb_per_cpu *cpu_buffer)
{
	local_set(&cpu_buffer->tail_page->page->commit,
		  cpu_buffer->tail_page->write);
	simple_rb_meta_inc(cpu_buffer->meta->entries, 1);

	/*
	 * Paired with simple_rb_enable_tracing() to ensure data is
	 * written to the ring-buffer before teardown.
	 */
	smp_store_release(&cpu_buffer->status, SIMPLE_RB_READY);
}
EXPORT_SYMBOL_GPL(simple_ring_buffer_commit);

static u32 simple_rb_enable_tracing(struct simple_rb_per_cpu *cpu_buffer, bool enable)
{
	u32 prev_status;

	if (enable)
		return cmpxchg(&cpu_buffer->status, SIMPLE_RB_UNAVAILABLE, SIMPLE_RB_READY);

	/* Wait for the buffer to be released */
	do {
		prev_status = cmpxchg_acquire(&cpu_buffer->status,
					      SIMPLE_RB_READY,
					      SIMPLE_RB_UNAVAILABLE);
	} while (prev_status == SIMPLE_RB_WRITING);

	return prev_status;
}

/**
 * simple_ring_buffer_reset - Reset @cpu_buffer
 * @cpu_buffer: A simple_rb_per_cpu
 *
 * This will not clear the content of the data, only reset counters and pointers
 *
 * Returns 0 on success or -ENODEV if @cpu_buffer was unloaded.
 */
int simple_ring_buffer_reset(struct simple_rb_per_cpu *cpu_buffer)
{
	struct simple_buffer_page *bpage;
	u32 prev_status;
	int ret;

	if (!simple_rb_loaded(cpu_buffer))
		return -ENODEV;

	prev_status = simple_rb_enable_tracing(cpu_buffer, false);

	ret = simple_rb_find_head(cpu_buffer);
	if (ret)
		return ret;

	bpage = cpu_buffer->tail_page = cpu_buffer->head_page;
	do {
		simple_bpage_reset(bpage);
		bpage = simple_bpage_next_page(bpage);
	} while (bpage != cpu_buffer->head_page);

	simple_bpage_reset(cpu_buffer->reader_page);

	cpu_buffer->last_overrun = 0;
	cpu_buffer->write_stamp = 0;

	cpu_buffer->meta->reader.read = 0;
	cpu_buffer->meta->reader.lost_events = 0;
	cpu_buffer->meta->entries = 0;
	cpu_buffer->meta->overrun = 0;
	cpu_buffer->meta->read = 0;
	cpu_buffer->meta->pages_lost = 0;
	cpu_buffer->meta->pages_touched = 0;

	if (prev_status == SIMPLE_RB_READY)
		simple_rb_enable_tracing(cpu_buffer, true);

	return 0;
}
EXPORT_SYMBOL_GPL(simple_ring_buffer_reset);

/**
 * simple_ring_buffer_init - Init @cpu_buffer based on @desc
 * @cpu_buffer:	A simple_rb_per_cpu buffer to init, allocated by the caller.
 * @bpages:	Array of simple_buffer_pages, with as many elements as @desc->nr_page_va
 * @desc:	A ring_buffer_desc
 *
 * Returns 0 on success or -EINVAL if the content of @desc is invalid
 */
int simple_ring_buffer_init(struct simple_rb_per_cpu *cpu_buffer, struct simple_buffer_page *bpages,
			    const struct ring_buffer_desc *desc)
{
	struct simple_buffer_page *bpage = bpages;
	int i;

	/* At least 1 reader page and two pages in the ring-buffer */
	if (desc->nr_page_va < 3)
		return -EINVAL;

	memset(cpu_buffer, 0, sizeof(*cpu_buffer));

	cpu_buffer->bpages = bpages;

	cpu_buffer->meta = (void *)desc->meta_va;
	memset(cpu_buffer->meta, 0, sizeof(*cpu_buffer->meta));
	cpu_buffer->meta->meta_page_size = PAGE_SIZE;
	cpu_buffer->meta->nr_subbufs = cpu_buffer->nr_pages;

	/* The reader page is not part of the ring initially */
	simple_bpage_init(bpage, desc->page_va[0]);
	bpage->id = 0;

	cpu_buffer->nr_pages = 1;

	cpu_buffer->reader_page = bpage;
	cpu_buffer->tail_page = bpage + 1;
	cpu_buffer->head_page = bpage + 1;

	for (i = 1; i < desc->nr_page_va; i++) {
		simple_bpage_init(++bpage, desc->page_va[i]);

		bpage->link.next = &(bpage + 1)->link;
		bpage->link.prev = &(bpage - 1)->link;
		bpage->id = i;

		cpu_buffer->nr_pages = i + 1;
	}

	/* Close the ring */
	bpage->link.next = &cpu_buffer->tail_page->link;
	cpu_buffer->tail_page->link.prev = &bpage->link;

	/* The last init'ed page points to the head page */
	simple_bpage_set_head_link(bpage);

	return 0;
}
EXPORT_SYMBOL_GPL(simple_ring_buffer_init);

/**
 * simple_ring_buffer_unload - Prepare @cpu_buffer for deletion
 * @cpu_buffer:	A simple_rb_per_cpu that will be deleted.
 */
void simple_ring_buffer_unload(struct simple_rb_per_cpu *cpu_buffer)
{
	if (!simple_rb_loaded(cpu_buffer))
		return;

	simple_rb_enable_tracing(cpu_buffer, false);

	cpu_buffer->bpages = NULL;
}
EXPORT_SYMBOL_GPL(simple_ring_buffer_unload);

/**
 * simple_ring_buffer_enable_tracing - Enable or disable writing to @cpu_buffer
 * @cpu_buffer: A simple_rb_per_cpu
 * @enable:	True to enable tracing, False to disable it
 *
 * Returns 0 on success or -ENODEV if @cpu_buffer was unloaded
 */
int simple_ring_buffer_enable_tracing(struct simple_rb_per_cpu *cpu_buffer, bool enable)
{
	if (!simple_rb_loaded(cpu_buffer))
		return -ENODEV;

	simple_rb_enable_tracing(cpu_buffer, enable);

	return 0;
}
EXPORT_SYMBOL_GPL(simple_ring_buffer_enable_tracing);