Re: Queston about io_uring_flush

[Pavel Begunkov <asml.silence@gmail.com>]

On 04/02/2021 09:31, Hao Xu wrote:
> Hi all,
> Sorry for disturb all of you. Here comes my question.
> When we close a uring file, we go into io_uring_flush(),
> there is codes at the end:
> 
> if (!(ctx->flags & IORING_SETUP_SQPOLL) || ctx->sqo_task == current)
>    io_uring_del_task_file(file);
> 
> My understanding, this is to delete the ctx(associated with the uring
> file) from current->io_uring->xa.
> I'm thinking of this scenario: the task to close uring file is not the
> one which created the uring file.
> Then it doesn't make sense to delete the uring file from current->io_uring->xa. It should be "delete uring file from
> ctx->sqo_task->io_uring->xa" instead.

1. It's not only about created or not, look for
io_uring_add_task_file() call sites.

2. io_uring->xa is basically a map from task to used by it urings.
Every user task should clean only its own context (SQPOLL task is
a bit different), it'll be hell bunch of races otherwise.

3. If happens that it's closed by a task that has nothing to do
with this ctx, then it won't find anything in its
task->io_uring->xa, and so won't delete anything, and that's ok.
io_uring->xa of sqo_task will be cleaned by sqo_task, either
on another close() or on exit() (see io_uring_files_cancel).

4. There is a bunch of cases where that scheme doesn't behave
nice, but at least should not leak/fault when all related tasks
are killed.

-- 
Pavel Begunkov