Re: [PATCH] file: always lock position

[Jens Axboe <axboe@kernel.dk>]

On 7/24/23 12:01?PM, Linus Torvalds wrote:
> On Mon, 24 Jul 2023 at 10:46, Christian Brauner <brauner@kernel.org> wrote:
>>
>> I don't think we do but it's something to keep in mind with async io
>> interfaces where the caller is free to create other threads after having
>> registered a request. Depending on how file references are done things
>> can get tricky easily.
> 
> Honestly, by now, the io_uring code had *better* understand that it
> needs to act exactly like a user thread.
> 
> Anything else is simply not acceptable. io_uring has been a huge pain,
> and the only thing that salvaged the horror was that the io_uring
> async code should now *always* be done as a real thread.
> 
> If io_uring does something from truly async context (ie interrupts,
> not io_uring threads), then io_uring had better be very *very*
> careful.
> 
> And any kind of "from kthread context" is not acceptable. We've been
> there, done that, and have the battle scars. Never again.
> 
> So the absolutely *only* acceptable context is "I'm a real
> io_uringthread that looks exactly like a user thread in every which
> way, except I never return to user space".
> 
> And if io_uring does absolutely _anything_ to file descriptors from
> any other context, it needs to be fixed *NOW*.

io_uring never does that isn't the original user space creator task, or
from the io-wq workers that it may create. Those are _always_ normal
threads. There's no workqueue/kthread usage for IO or file
getting/putting/installing/removing etc.

-- 
Jens Axboe