Re: [PATCH] printk: mark racy data block id accesses in the ringbuffer

[John Ogness]

On 2026-10-11, Nguyen Ngoc Thang <ngocthang2710.1999@xxxxxxxxx> wrote:
> data_alloc() stores the descriptor ID into a newly reserved data block
> while data_make_reusable(), running from data_push_tail() on another CPU,
> may still be loading the ID from the same area. The code relies on this
> race being harmless: a stale or garbage ID simply fails the descriptor
> lookup or the blk_lpos check. But the accesses are plain, so KCSAN
> reports them, and the compiler is free to tear or refetch the ID.
>
> Use READ_ONCE()/WRITE_ONCE() for every access to the block ID that can
> overlap with the lockless reader. No functional change intended.

READ_ONCE()/WRITE_ONCE() could cause minor side-effects as it prohibits
optimizations that do not need to be prohibited. These would not cause a
problem, but they are unnecessary. The data race is allowed.

How about using data_race() instead?

	id = data_race(blk->id);

	data_race(blk->id = id);

John Ogness