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From: Jann Horn <[email protected]>
To: Jens Axboe <[email protected]>
Cc: io-uring <[email protected]>,
	Glauber Costa <[email protected]>,
	Peter Zijlstra <[email protected]>,
	Pavel Begunkov <[email protected]>
Subject: Re: [PATCH 7/9] io_uring: add per-task callback handler
Date: Thu, 20 Feb 2020 23:56:25 +0100	[thread overview]
Message-ID: <CAG48ez3d-HD2_tC+kUwS4ch4ifd=ota27J_2wZ7ovGFT5R7Hnw@mail.gmail.com> (raw)
In-Reply-To: <CAG48ez1sQi7ntGnLxyo9X_642-wr55+Kn662XyyEYGLyi0iLwQ@mail.gmail.com>

On Thu, Feb 20, 2020 at 11:02 PM Jann Horn <[email protected]> wrote:
> On Thu, Feb 20, 2020 at 9:32 PM Jens Axboe <[email protected]> wrote:
> >
> > For poll requests, it's not uncommon to link a read (or write) after
> > the poll to execute immediately after the file is marked as ready.
> > Since the poll completion is called inside the waitqueue wake up handler,
> > we have to punt that linked request to async context. This slows down
> > the processing, and actually means it's faster to not use a link for this
> > use case.
> >
> > We also run into problems if the completion_lock is contended, as we're
> > doing a different lock ordering than the issue side is. Hence we have
> > to do trylock for completion, and if that fails, go async. Poll removal
> > needs to go async as well, for the same reason.
> >
> > eventfd notification needs special case as well, to avoid stack blowing
> > recursion or deadlocks.
> >
> > These are all deficiencies that were inherited from the aio poll
> > implementation, but I think we can do better. When a poll completes,
> > simply queue it up in the task poll list. When the task completes the
> > list, we can run dependent links inline as well. This means we never
> > have to go async, and we can remove a bunch of code associated with
> > that, and optimizations to try and make that run faster. The diffstat
> > speaks for itself.
> [...]
> > -static void io_poll_trigger_evfd(struct io_wq_work **workptr)
> > +static void io_poll_task_func(struct callback_head *cb)
> >  {
> > -       struct io_kiocb *req = container_of(*workptr, struct io_kiocb, work);
> > +       struct io_kiocb *req = container_of(cb, struct io_kiocb, sched_work);
> > +       struct io_kiocb *nxt = NULL;
> >
> [...]
> > +       io_poll_task_handler(req, &nxt);
> > +       if (nxt)
> > +               __io_queue_sqe(nxt, NULL);
>
> This can now get here from anywhere that calls schedule(), right?
> Which means that this might almost double the required kernel stack
> size, if one codepath exists that calls schedule() while near the
> bottom of the stack and another codepath exists that goes from here
> through the VFS and again uses a big amount of stack space?

Oh, I think this also implies that any mutex reachable via any of the
nonblocking uring ops nests inside any mutex under which we happen to
schedule(), right? I wonder whether that's going to cause deadlocks...

For example, FUSE's ->read_iter() can call fuse_direct_io(), which can
call inode_lock() and then call fuse_sync_writes() under the inode
lock, which can wait_event(), which can schedule(); and if uring then
from schedule() calls ->read_iter() again, you could reach
inode_lock() on the same inode again, causing a deadlock, I think?

  parent reply	other threads:[~2020-02-20 22:56 UTC|newest]

Thread overview: 53+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-20 20:31 [PATCHSET 0/9] io_uring: use polled async retry Jens Axboe
2020-02-20 20:31 ` [PATCH 1/9] io_uring: consider any io_read/write -EAGAIN as final Jens Axboe
2020-02-20 20:31 ` [PATCH 2/9] io_uring: io_accept() should hold on to submit reference on retry Jens Axboe
2020-02-20 20:31 ` [PATCH 3/9] sched: move io-wq/workqueue worker sched in/out into helpers Jens Axboe
2020-02-20 20:31 ` [PATCH 4/9] task_work_run: don't take ->pi_lock unconditionally Jens Axboe
2020-02-20 20:31 ` [PATCH 5/9] kernel: abstract out task work helpers Jens Axboe
2020-02-20 21:07   ` Peter Zijlstra
2020-02-20 21:08     ` Jens Axboe
2020-02-20 20:31 ` [PATCH 6/9] sched: add a sched_work list Jens Axboe
2020-02-20 21:17   ` Peter Zijlstra
2020-02-20 21:53     ` Jens Axboe
2020-02-20 22:02       ` Jens Axboe
2020-02-20 20:31 ` [PATCH 7/9] io_uring: add per-task callback handler Jens Axboe
2020-02-20 22:02   ` Jann Horn
2020-02-20 22:14     ` Jens Axboe
2020-02-20 22:18       ` Jens Axboe
2020-02-20 22:25         ` Jann Horn
2020-02-20 22:23       ` Jens Axboe
2020-02-20 22:38         ` Jann Horn
2020-02-20 22:56           ` Jens Axboe
2020-02-20 22:58             ` Jann Horn
2020-02-20 23:02               ` Jens Axboe
2020-02-20 22:23       ` Jann Horn
2020-02-20 23:00         ` Jens Axboe
2020-02-20 23:12           ` Jann Horn
2020-02-20 23:22             ` Jens Axboe
2020-02-21  1:29               ` Jann Horn
2020-02-21 17:32                 ` Jens Axboe
2020-02-21 19:24                   ` Jann Horn
2020-02-21 20:18                     ` Jens Axboe
2020-02-20 22:56     ` Jann Horn [this message]
2020-02-21 10:47     ` Peter Zijlstra
2020-02-21 14:49       ` Jens Axboe
2020-02-21 15:02         ` Jann Horn
2020-02-21 16:12           ` Peter Zijlstra
2020-02-21 16:23         ` Peter Zijlstra
2020-02-21 20:13           ` Jens Axboe
2020-02-21 13:51   ` Pavel Begunkov
2020-02-21 14:50     ` Jens Axboe
2020-02-21 18:30       ` Pavel Begunkov
2020-02-21 19:10         ` Jens Axboe
2020-02-21 19:22           ` Pavel Begunkov
2020-02-23  6:00           ` Jens Axboe
2020-02-23  6:26             ` Jens Axboe
2020-02-23 11:02               ` Pavel Begunkov
2020-02-23 14:49                 ` Jens Axboe
2020-02-23 14:58                   ` Jens Axboe
2020-02-23 15:07                     ` Jens Axboe
2020-02-23 18:04                       ` Pavel Begunkov
2020-02-23 18:06                         ` Jens Axboe
2020-02-23 17:55                   ` Pavel Begunkov
2020-02-20 20:31 ` [PATCH 8/9] io_uring: mark requests that we can do poll async in io_op_defs Jens Axboe
2020-02-20 20:31 ` [PATCH 9/9] io_uring: use poll driven retry for files that support it Jens Axboe

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