Re: [PATCH] ring-buffer: serialize read-page order with subbuffer resize

[Steven Rostedt <rostedt@goodmis.org>]

On Tue, 30 Jun 2026 13:45:05 -0700
Yousef Alhouseen <alhouseenyousef@gmail.com> wrote:

> One issue turned up while checking the suggested locking:
> ring_buffer_subbuf_order_set() writes buffer->subbuf_order before
> taking reader_lock and never takes cpu_buffer->lock. An allocator can
> therefore take cpu_buffer->lock after the new order is published but
> before resize clears the old-order free_page, tag that old page with
> the new order, and return it.
> 
> I can keep the allocations outside buffer->mutex and hold the mutex
> only while snapshotting subbuf_order and taking or returning
> free_page. That removes allocation from the critical section and
> serializes the order/free-page pair with resize. Would you prefer
> that, or should free_page and the order transition be synchronized
> another way?

Nothing should be reading when the subbuf_order is being updated. Let's add
a flag to state that it's being updated, and make all reads simply fail
during that time.

-- Steve