Re: [PATCH] ring-buffer: Take cpus_read_lock() when changing the subbuf order

[Vincent Donnefort <vdonnefort@google.com>]

On Mon, Sep 14, 2026 at 07:28:43PM +0100, David Carlier wrote:
> A CPU coming online during ring_buffer_subbuf_order_set() can be added

nit: a "new" CPU or "A CPU we've never seen before"?

Offlined CPUs aren't removed from ->cpumask.

> to buffer->cpumask after the allocation loop, and the install loop then
> uses its empty new_pages list, corrupting the buffer.
> 
> Take cpus_read_lock() as ring_buffer_resize() does.
> 
> Fixes: f9b94daa542a ("ring-buffer: Set new size of the ring buffer sub page")
> Cc: stable@vger.kernel.org
> Signed-off-by: David Carlier <devnexen@gmail.com>
> ---
>  kernel/trace/ring_buffer.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 04bb94c29f58..445931bc9ae0 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -7464,6 +7464,12 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
>  	if (psize > RB_WRITE_MASK + 1)
>  		return -EINVAL;
>  
> +	/*
> +	 * Keep CPUs from coming online while changing the order to
> +	 * synchronize with new per CPU buffers being created.
> +	 */

Can we drop this comment? cpus_read_lock is self-explanatory.

> +	guard(cpus_read_lock)();
> +
>  	/* prevent another thread from changing buffer sizes */
>  	guard(mutex)(&buffer->mutex);
>  
> -- 
> 2.55.0
> 

-- 
Vincent