Re: [PATCH v1] io_uring/kbuf: use WRITE_ONCE() for userspace-shared buffer ring fields

[Jens Axboe <axboe@kernel.dk>]

On 12/5/25 9:55 AM, Caleb Sander Mateos wrote:
> On Thu, Dec 4, 2025 at 3:55?PM Joanne Koong <joannelkoong@gmail.com> wrote:
>>
>> buf->addr and buf->len reside in memory shared with userspace. They
>> should be written with WRITE_ONCE() to guarantee atomic stores and
>> prevent tearing or other unsafe compiler optimizations.
> 
> I considered this too, but I'm not sure it's necessary. A correctly
> written userspace program won't access these fields concurrently with
> the kernel, right? I'm not too familiar with the buffer ring UAPI, but
> I would assume userspace is notified somehow (via a posted CQE?) that
> the io_uring_buf slots have been consumed and are available to reuse.
> In that case, a torn store here would only be observable to a buggy
> userspace program, so I don't think we need to add WRITE_ONCE().

I agree, see my reply from a few minutes ago. But I do think it serves a
documentation purpose ("shared memory is always read/write once"), which
is why I applied it.

-- 
Jens Axboe