On 10/16/20 5:38 PM, Thomas Gleixner wrote:
> On Fri, Oct 16 2020 at 17:13, Jens Axboe wrote:
>> /**
>> * task_work_add - ask the @task to execute @work->func()
>> * @task: the task which should run the callback
>> * @work: the callback to run
>> * @notify: how to notify the targeted task
>> *
>> * Queue @work for task_work_run() below and notify the @task if @notify
>> * is @TWA_RESUME or @TWA_SIGNAL. @TWA_SIGNAL work like signals, in that the
>
> s/the//
Thanks, good catch.
>> * it will interrupt the targeted task and run the task_work. @TWA_RESUME
>> * work is run only when the task exits the kernel and returns to user mode.
>> * Fails if the @task is exiting/exited and thus it can't process this @work.
>> * Otherwise @work->func() will be called when the @task returns from kernel
>> * mode or exits.
>
> Yes, that makes a lot more sense.
>
> What's still lacking is a description of the return value and how to act
> upon it.
That's really up to the caller. But we should add some explanation of
that. Most callers use some alternative if the task is exiting, like
using a work queue for example.
> Most of the call sites ignore it, some are acting upon it but I can't
If you know the task isn't exiting, then yeah you can ignore it. But
seems a bit dicey...
> make any sense of these actions:
>
> fs/io_uring.c- notify = 0;
> fs/io_uring.c- if (!(ctx->flags & IORING_SETUP_SQPOLL) &&
> twa_signal_ok)
> fs/io_uring.c- notify = TWA_SIGNAL;
> fs/io_uring.c-
> fs/io_uring.c: ret = task_work_add(tsk, &req->task_work, notify);
> fs/io_uring.c- if (!ret)
> fs/io_uring.c- wake_up_process(tsk);
>
> ???
>
> fs/io_uring.c- if (unlikely(ret)) {
> fs/io_uring.c- struct task_struct *tsk;
> fs/io_uring.c-
> fs/io_uring.c- init_task_work(&req->task_work,
> io_req_task_cancel);
> fs/io_uring.c- tsk = io_wq_get_task(req->ctx->io_wq);
> fs/io_uring.c: task_work_add(tsk, &req->task_work, 0);
> fs/io_uring.c- wake_up_process(tsk);
>
> yet more magic wakeup.
It's not magic, but probably needs a comment... If we fail, that task is
exiting. But we know we have our io-wq threads, so we use that as a
fallback. Not really expected in the fast path.
> fs/io_uring.c-
> fs/io_uring.c- init_task_work(&req->task_work, io_req_task_submit);
> fs/io_uring.c- percpu_ref_get(&req->ctx->refs);
> fs/io_uring.c-
> fs/io_uring.c- /* submit ref gets dropped, acquire a new one */
> fs/io_uring.c- refcount_inc(&req->refs);
> fs/io_uring.c: ret = io_req_task_work_add(req, true);
> fs/io_uring.c- if (unlikely(ret)) {
> fs/io_uring.c- struct task_struct *tsk;
> fs/io_uring.c-
> fs/io_uring.c- /* queue just for cancelation */
> fs/io_uring.c- init_task_work(&req->task_work,
> io_req_task_cancel);
> fs/io_uring.c- tsk = io_wq_get_task(req->ctx->io_wq);
> fs/io_uring.c: task_work_add(tsk, &req->task_work, 0);
> fs/io_uring.c- wake_up_process(tsk);
>
> Ditto. Why the heck is this wakeup making any sense? The initial
> task_work_add() within io_req_task_work_add() failed already ...
Right, but we're using a new task for this. And that task is a kthread
that we manage, hence no notification is needed outside of just waking
it up.
--
Jens Axboe
