Re: [PATCH v5 04/10] ring-buffer: Fix subbuf resize race with ring buffer readers

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Thu, 13 Aug 2026 14:11:46 +0100
Vincent Donnefort <vdonnefort@google.com> wrote:

> trace_buffer subbuf_size is read lockless in ring_buffer_read_page() and
> ring_buffer_read_start(), while it can simultaneously be resized with
> ring_buffer_subbuf_order_set().
> 
> Instead of trace_buffer::subbuf_size, use bpage::order in
> ring_buffer_read_start() and ring_buffer_read_page().
> 
> In ring_buffer_read_start(), even with resize_disabled, there is still a
> possibility of a race with a buffer modification. Hold the trace_buffer
> mutex to synchronise with any pending ring buffer order modification.
> 
> trace_buffer::subbuf_size is now actually useless, remove it. Also,
> create accessors rb_subbuf_capacity() and rb_page_capacity() which
> return the actual size available for storing events, while
> rb_subbuf_size() returns the actual subbuf page-size.
> 
> Fixes: f9b94daa542a ("ring-buffer: Set new size of the ring buffer sub page")
> Reported-by: Sashiko <sashiko-bot@kernel.org>

Can you add Closes: tag with Sashiko's url?

The code looks good to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Thanks,

> Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index b6fa258aafe2..ec520c72124e 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -391,6 +391,17 @@ static __always_inline unsigned int rb_page_size(struct buffer_page *bpage)
>  	return rb_data_page_size(bpage->page);
>  }
>  
> +/**
> + * rb_page_capacity - Get the capacity of a buffer page
> + * @bpage:	The buffer page
> + *
> + * Return: The maximum size available for events in the given buffer page.
> + */
> +static __always_inline unsigned int rb_page_capacity(struct buffer_page *bpage)
> +{
> +	return (PAGE_SIZE << bpage->order) - BUF_PAGE_HDR_SIZE;
> +}
> +
>  static void free_buffer_page(struct buffer_page *bpage)
>  {
>  	/* Range pages are not to be freed */
> @@ -586,11 +597,42 @@ struct trace_buffer {
>  
>  	struct ring_buffer_meta		*meta;
>  
> -	unsigned int			subbuf_size;
>  	unsigned int			subbuf_order;
>  	unsigned int			max_data_size;
>  };
>  
> +static __always_inline unsigned int rb_subbuf_size(struct trace_buffer *buffer)
> +{
> +	return PAGE_SIZE << buffer->subbuf_order;
> +}
> +
> +/**
> + * rb_subbuf_capacity - Get the capacity of a subbuffer
> + * @buffer:	A trace buffer
> + *
> + * Unsafe to use without holding trace_buffer::mutex or with resizing enabled.
> + * Consider rb_page_capacity() instead.
> + *
> + * Return: The maximum size available for events in a trace buffer subbuffer.
> + */
> +static __always_inline unsigned int rb_subbuf_capacity(struct trace_buffer *buffer)
> +{
> +	return rb_subbuf_size(buffer) - BUF_PAGE_HDR_SIZE;
> +}
> +
> +/**
> + * rb_subbuf_start - Get the start address of a subbuffer
> + * @buffer:	A trace buffer
> + * @addr:	An address of an event on a subbuffer
> + *
> + * Return: The start of the subbuffer for where @addr sits
> + */
> +static __always_inline
> +unsigned long rb_subbuf_start(struct trace_buffer *buffer, unsigned long addr)
> +{
> +	return addr & ~((unsigned long)(rb_subbuf_size(buffer) - 1));
> +}
> +
>  struct ring_buffer_iter {
>  	struct ring_buffer_per_cpu	*cpu_buffer;
>  	unsigned long			head;
> @@ -630,7 +672,7 @@ int ring_buffer_print_page_header(struct trace_buffer *buffer, struct trace_seq
>  	trace_seq_printf(s, "\tfield: char data;\t"
>  			 "offset:%u;\tsize:%u;\tsigned:%u;\n",
>  			 (unsigned int)offsetof(typeof(field), data),
> -			 (unsigned int)(buffer ? buffer->subbuf_size :
> +			 (unsigned int)(buffer ? rb_subbuf_capacity(buffer) :
>  						 PAGE_SIZE - BUF_PAGE_HDR_SIZE),
>  			 (unsigned int)is_signed_type(char));
>  
> @@ -1620,7 +1662,7 @@ rb_range_align_subbuf(unsigned long addr, int subbuf_size, int nr_subbufs)
>   */
>  static void *rb_range_meta(struct trace_buffer *buffer, int nr_pages, int cpu)
>  {
> -	int subbuf_size = buffer->subbuf_size + BUF_PAGE_HDR_SIZE;
> +	int subbuf_size = rb_subbuf_size(buffer);
>  	struct ring_buffer_cpu_meta *meta;
>  	struct ring_buffer_meta *bmeta;
>  	unsigned long ptr;
> @@ -2432,8 +2474,8 @@ static int __rb_allocate_pages(struct ring_buffer_per_cpu *cpu_buffer,
>  			bpage->id = i + 1;
>  			cpu_buffer->subbuf_ids[i + 1] = bpage;
>  		} else {
> -			int order = cpu_buffer->buffer->subbuf_order;
> -			bpage->page = alloc_cpu_data(cpu_buffer->cpu, order);
> +			bpage->page = alloc_cpu_data(cpu_buffer->cpu,
> +						     cpu_buffer->buffer->subbuf_order);
>  			if (!bpage->page)
>  				goto free_pages;
>  		}
> @@ -2556,8 +2598,7 @@ rb_allocate_cpu_buffer(struct trace_buffer *buffer, long nr_pages, int cpu)
>  		bpage->range = 1;
>  		cpu_buffer->subbuf_ids[0] = bpage;
>  	} else {
> -		int order = cpu_buffer->buffer->subbuf_order;
> -		bpage->page = alloc_cpu_data(cpu, order);
> +		bpage->page = alloc_cpu_data(cpu, bpage->order);
>  		if (!bpage->page)
>  			goto fail_free_reader;
>  	}
> @@ -2731,10 +2772,9 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
>  
>  	buffer->subbuf_order = order;
>  	subbuf_size = (PAGE_SIZE << order);
> -	buffer->subbuf_size = subbuf_size - BUF_PAGE_HDR_SIZE;
>  
>  	/* Max payload is buffer page size - header (8bytes) */
> -	buffer->max_data_size = buffer->subbuf_size - (sizeof(u32) * 2);
> +	buffer->max_data_size = rb_subbuf_capacity(buffer) - (sizeof(u32) * 2);
>  
>  	buffer->flags = flags;
>  	buffer->clock = trace_clock_local;
> @@ -2818,9 +2858,8 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
>  		if (nr_pages < 2)
>  			goto fail_free_buffers;
>  	} else {
> -
>  		/* need at least two pages */
> -		nr_pages = DIV_ROUND_UP(size, buffer->subbuf_size);
> +		nr_pages = DIV_ROUND_UP(size, rb_subbuf_capacity(buffer));
>  		if (nr_pages < 2)
>  			nr_pages = 2;
>  	}
> @@ -3203,7 +3242,7 @@ static void update_pages_handler(struct work_struct *work)
>   * @size: the new size.
>   * @cpu_id: the cpu buffer to resize
>   *
> - * Minimum size is 2 * buffer->subbuf_size.
> + * Minimum size is 2 * rb_subbuf_capacity(buffer).
>   *
>   * Returns 0 on success and < 0 on failure.
>   */
> @@ -3225,12 +3264,6 @@ int ring_buffer_resize(struct trace_buffer *buffer, unsigned long size,
>  	    !cpumask_test_cpu(cpu_id, buffer->cpumask))
>  		return 0;
>  
> -	nr_pages = DIV_ROUND_UP(size, buffer->subbuf_size);
> -
> -	/* we need a minimum of two pages */
> -	if (nr_pages < 2)
> -		nr_pages = 2;
> -
>  	/*
>  	 * Keep CPUs from coming online while resizing to synchronize
>  	 * with new per CPU buffers being created.
> @@ -3241,6 +3274,12 @@ int ring_buffer_resize(struct trace_buffer *buffer, unsigned long size,
>  	mutex_lock(&buffer->mutex);
>  	atomic_inc(&buffer->resizing);
>  
> +	nr_pages = DIV_ROUND_UP(size, rb_subbuf_capacity(buffer));
> +
> +	/* we need a minimum of two pages */
> +	if (nr_pages < 2)
> +		nr_pages = 2;
> +
>  	if (cpu_id == RING_BUFFER_ALL_CPUS) {
>  		/*
>  		 * Don't succeed if resizing is disabled, as a reader might be
> @@ -3513,7 +3552,7 @@ rb_event_index(struct ring_buffer_per_cpu *cpu_buffer, struct ring_buffer_event
>  {
>  	unsigned long addr = (unsigned long)event;
>  
> -	addr &= (PAGE_SIZE << cpu_buffer->buffer->subbuf_order) - 1;
> +	addr &= (unsigned long)rb_subbuf_size(cpu_buffer->buffer) - 1;
>  
>  	return addr - BUF_PAGE_HDR_SIZE;
>  }
> @@ -3755,8 +3794,8 @@ static inline void
>  rb_reset_tail(struct ring_buffer_per_cpu *cpu_buffer,
>  	      unsigned long tail, struct rb_event_info *info)
>  {
> -	unsigned long bsize = READ_ONCE(cpu_buffer->buffer->subbuf_size);
>  	struct buffer_page *tail_page = info->tail_page;
> +	unsigned long bsize = rb_page_capacity(tail_page);
>  	struct ring_buffer_event *event;
>  	unsigned long length = info->length;
>  
> @@ -4101,8 +4140,7 @@ rb_try_to_discard(struct ring_buffer_per_cpu *cpu_buffer,
>  
>  	new_index = rb_event_index(cpu_buffer, event);
>  	old_index = new_index + rb_event_ts_length(event);
> -	addr = (unsigned long)event;
> -	addr &= ~((PAGE_SIZE << cpu_buffer->buffer->subbuf_order) - 1);
> +	addr = rb_subbuf_start(cpu_buffer->buffer, (unsigned long)event);
>  
>  	bpage = READ_ONCE(cpu_buffer->tail_page);
>  
> @@ -4767,7 +4805,7 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
>  	tail = write - info->length;
>  
>  	/* See if we shot pass the end of this buffer page */
> -	if (unlikely(write > cpu_buffer->buffer->subbuf_size)) {
> +	if (unlikely(write > rb_page_capacity(tail_page))) {
>  		check_buffer(cpu_buffer, info, CHECK_FULL_PAGE);
>  		return rb_move_tail(cpu_buffer, tail, info);
>  	}
> @@ -5012,7 +5050,7 @@ rb_decrement_entry(struct ring_buffer_per_cpu *cpu_buffer,
>  	struct buffer_page *bpage = cpu_buffer->commit_page;
>  	struct buffer_page *start;
>  
> -	addr &= ~((PAGE_SIZE << cpu_buffer->buffer->subbuf_order) - 1);
> +	addr = rb_subbuf_start(cpu_buffer->buffer, addr);
>  
>  	/* Do the likely case first */
>  	if (likely(bpage->page == (void *)addr)) {
> @@ -5799,7 +5837,6 @@ static struct buffer_page *
>  __rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
>  {
>  	int max_loops = cpu_buffer->ring_meta ? cpu_buffer->nr_pages : 3;
> -	unsigned long bsize = READ_ONCE(cpu_buffer->buffer->subbuf_size);
>  	struct buffer_page *reader = NULL;
>  	unsigned long overwrite;
>  	unsigned long flags;
> @@ -5947,7 +5984,7 @@ __rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
>  #define USECS_WAIT	1000000
>          for (nr_loops = 0; nr_loops < USECS_WAIT; nr_loops++) {
>  		/* If the write is past the end of page, a writer is still updating it */
> -		if (likely(!reader || rb_page_write(reader) <= bsize))
> +		if (likely(!reader || rb_page_write(reader) <= rb_page_capacity(reader)))
>  			break;
>  
>  		udelay(1);
> @@ -6380,36 +6417,44 @@ EXPORT_SYMBOL_GPL(ring_buffer_consume);
>  struct ring_buffer_iter *
>  ring_buffer_read_start(struct trace_buffer *buffer, int cpu, gfp_t flags)
>  {
> +	struct ring_buffer_iter *iter __free(kfree) = kzalloc_obj(*iter, flags);
>  	struct ring_buffer_per_cpu *cpu_buffer;
> -	struct ring_buffer_iter *iter;
> +
> +	if (!iter)
> +		return NULL;
>  
>  	if (!cpumask_test_cpu(cpu, buffer->cpumask))
>  		return NULL;
>  
> -	iter = kzalloc_obj(*iter, flags);
> -	if (!iter)
> -		return NULL;
> -
> -	/* Holds the entire event: data and meta data */
> -	iter->event_size = buffer->subbuf_size;
> -	iter->event = kmalloc(iter->event_size, flags);
> -	if (!iter->event) {
> -		kfree(iter);
> -		return NULL;
> -	}
> -
>  	cpu_buffer = buffer->buffers[cpu];
>  
> -	iter->cpu_buffer = cpu_buffer;
> +	/*
> +	 * Only KDB is using GFP_ATOMIC, for the others, lock the buffer to
> +	 * prevent concurrent resizing.
> +	 */
> +	if (gfpflags_allow_blocking(flags))
> +		mutex_lock(&buffer->mutex);
>  
>  	atomic_inc(&cpu_buffer->resize_disabled);
>  
> +	if (gfpflags_allow_blocking(flags))
> +		mutex_unlock(&buffer->mutex);
> +
> +	/* Holds the entire event: data and meta data. */
> +	iter->event_size = rb_page_capacity(READ_ONCE(cpu_buffer->reader_page));
> +	iter->event = kmalloc(iter->event_size, flags);
> +	if (!iter->event) {
> +		atomic_dec(&cpu_buffer->resize_disabled);
> +		return NULL;
> +	}
> +	iter->cpu_buffer = cpu_buffer;
> +
>  	guard(raw_spinlock_irqsave)(&cpu_buffer->reader_lock);
>  	arch_spin_lock(&cpu_buffer->lock);
>  	rb_iter_reset(iter);
>  	arch_spin_unlock(&cpu_buffer->lock);
>  
> -	return iter;
> +	return_ptr(iter);
>  }
>  EXPORT_SYMBOL_GPL(ring_buffer_read_start);
>  
> @@ -6463,7 +6508,7 @@ unsigned long ring_buffer_size(struct trace_buffer *buffer, int cpu)
>  	if (!cpumask_test_cpu(cpu, buffer->cpumask))
>  		return 0;
>  
> -	return buffer->subbuf_size * buffer->buffers[cpu]->nr_pages;
> +	return rb_subbuf_capacity(buffer) * buffer->buffers[cpu]->nr_pages;
>  }
>  EXPORT_SYMBOL_GPL(ring_buffer_size);
>  
> @@ -7094,15 +7139,15 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
>  	if (!data_page || !data_page->data)
>  		return -1;
>  
> -	if (data_page->order != buffer->subbuf_order)
> -		return -1;
> -
>  	dpage = data_page->data;
>  	if (!dpage)
>  		return -1;
>  
>  	guard(raw_spinlock_irqsave)(&cpu_buffer->reader_lock);
>  
> +	if (data_page->order != cpu_buffer->reader_page->order)
> +		return -1;
> +
>  	reader = rb_get_reader_page(cpu_buffer);
>  	if (!reader)
>  		return -1;
> @@ -7228,7 +7273,7 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
>  		 * missed events, then record it there.
>  		 */
>  		if (missed_events > 0 &&
> -		    buffer->subbuf_size - size >= sizeof(missed_events)) {
> +		    rb_page_capacity(reader) - size >= sizeof(missed_events)) {
>  			memcpy(&dpage->data[size], &missed_events,
>  			       sizeof(missed_events));
>  			local_add(RB_MISSED_STORED, &dpage->commit);
> @@ -7248,8 +7293,8 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
>  	/*
>  	 * This page may be off to user land. Zero it out here.
>  	 */
> -	if (size < buffer->subbuf_size)
> -		memset(&dpage->data[size], 0, buffer->subbuf_size - size);
> +	if (size < rb_page_capacity(reader))
> +		memset(&dpage->data[size], 0, rb_page_capacity(reader) - size);
>  
>  	return read;
>  }
> @@ -7275,7 +7320,7 @@ EXPORT_SYMBOL_GPL(ring_buffer_read_page_data);
>   */
>  int ring_buffer_subbuf_size_get(struct trace_buffer *buffer)
>  {
> -	return buffer->subbuf_size + BUF_PAGE_HDR_SIZE;
> +	return rb_subbuf_size(buffer);
>  }
>  EXPORT_SYMBOL_GPL(ring_buffer_subbuf_size_get);
>  
> @@ -7320,7 +7365,8 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
>  {
>  	struct ring_buffer_per_cpu *cpu_buffer;
>  	struct buffer_page *bpage, *tmp;
> -	int old_order, old_size;
> +	unsigned int old_capacity;
> +	int old_order;
>  	int nr_pages;
>  	int psize;
>  	int err;
> @@ -7329,9 +7375,6 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
>  	if (!buffer || order < 0)
>  		return -EINVAL;
>  
> -	if (buffer->subbuf_order == order)
> -		return 0;
> -
>  	psize = (1 << order) * PAGE_SIZE;
>  	if (psize <= BUF_PAGE_HDR_SIZE)
>  		return -EINVAL;
> @@ -7340,18 +7383,21 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
>  	if (psize > RB_WRITE_MASK + 1)
>  		return -EINVAL;
>  
> -	old_order = buffer->subbuf_order;
> -	old_size = buffer->subbuf_size;
> -
>  	/* prevent another thread from changing buffer sizes */
>  	guard(mutex)(&buffer->mutex);
> +
> +	old_order = buffer->subbuf_order;
> +	if (old_order == order)
> +		return 0;
> +
> +	old_capacity = rb_subbuf_capacity(buffer);
> +
>  	atomic_inc(&buffer->record_disabled);
>  
>  	/* Make sure all commits have finished */
>  	synchronize_rcu();
>  
>  	buffer->subbuf_order = order;
> -	buffer->subbuf_size = psize - BUF_PAGE_HDR_SIZE;
>  
>  	/* Make sure all new buffers are allocated, before deleting the old ones */
>  	for_each_buffer_cpu(buffer, cpu) {
> @@ -7367,8 +7413,8 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
>  		}
>  
>  		/* Update the number of pages to match the new size */
> -		nr_pages = old_size * buffer->buffers[cpu]->nr_pages;
> -		nr_pages = DIV_ROUND_UP(nr_pages, buffer->subbuf_size);
> +		nr_pages = old_capacity * buffer->buffers[cpu]->nr_pages;
> +		nr_pages = DIV_ROUND_UP(nr_pages, rb_subbuf_capacity(buffer));
>  
>  		/* we need a minimum of two pages */
>  		if (nr_pages < 2)
> @@ -7456,7 +7502,6 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
>  
>  error:
>  	buffer->subbuf_order = old_order;
> -	buffer->subbuf_size = old_size;
>  
>  	atomic_dec(&buffer->record_disabled);
>  
> @@ -7534,7 +7579,7 @@ static void rb_setup_ids_meta_page(struct ring_buffer_per_cpu *cpu_buffer,
>  
>  	meta->meta_struct_len = sizeof(*meta);
>  	meta->nr_subbufs = nr_subbufs;
> -	meta->subbuf_size = cpu_buffer->buffer->subbuf_size + BUF_PAGE_HDR_SIZE;
> +	meta->subbuf_size = rb_subbuf_size(cpu_buffer->buffer);
>  	meta->meta_page_size = meta->subbuf_size;
>  
>  	rb_update_meta_page(cpu_buffer);
> @@ -7896,7 +7941,7 @@ int ring_buffer_map_get_reader(struct trace_buffer *buffer, int cpu)
>  			 * missed events, then record it there.
>  			 */
>  			commit = rb_page_size(reader);
> -			if (buffer->subbuf_size - commit >= sizeof(missed_events)) {
> +			if (rb_subbuf_capacity(buffer) - commit >= sizeof(missed_events)) {
>  				memcpy(&dpage->data[commit], &missed_events,
>  				       sizeof(missed_events));
>  				local_add(RB_MISSED_STORED, &dpage->commit);
> @@ -7928,7 +7973,7 @@ int ring_buffer_map_get_reader(struct trace_buffer *buffer, int cpu)
>  out:
>  	/* Some archs do not have data cache coherency between kernel and user-space */
>  	flush_kernel_vmap_range(cpu_buffer->reader_page->page,
> -				buffer->subbuf_size + BUF_PAGE_HDR_SIZE);
> +				rb_subbuf_size(buffer));
>  
>  	rb_update_meta_page(cpu_buffer);
>  
> -- 
> 2.55.0.691.gc56d675ccc-goog
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>