Re: [PATCH] block: annotate lockless task ioprio accesses

[Damien Le Moal]

On 2026/10/19 14:18, Nguyen Ngoc Thang wrote:
> Hi Damien,
> 
> Thanks for looking. I don't think RCU is needed here, because the reader
> can only be the owner of the io_context:
> 
>  - The only path that drops the task's reference is exit_io_context(),
>    which the task runs on itself in do_exit() (and on the copy_process()
>    error path, where the task never ran). It clears task->io_context
>    under task_lock() and only then puts the ioc.
>  - So when __get_task_ioprio() runs lockless, p == current, and current
>    cannot be inside exit_io_context() at the same time. Once the pointer
>    is non-NULL it stays valid for as long as current is executing; the
>    ioc cannot be freed under it.
>  - For p != current the function already requires p->alloc_lock
>    (lockdep_assert_held()), and that is what serializes against
>    exit_io_context().
> 
> The only thing another task can change under the lockless reader is the
> NULL -> ioc transition and ->ioprio, both done by set_task_ioprio() under
> task_lock(), and that is exactly what the KCSAN report shows
> ("value changed: 0x0 -> 0xffff..."). The patch only needs to make that
> publication and the ->ioprio accesses well defined, not to extend the
> lifetime of the ioc.
> 
> I can add a sentence to the commit message and a short comment in
> __get_task_ioprio() stating this lifetime rule in a v2 if you think it
> helps.

Yes, a comment would be nice.

-- 
Damien Le Moal
Western Digital Research