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From: Jann Horn <jannh@google.com>
To: Jens Axboe <axboe@kernel.dk>
Cc: linux-aio@kvack.org, linux-block@vger.kernel.org,
	Linux API <linux-api@vger.kernel.org>,
	hch@lst.de, jmoyer@redhat.com, Avi Kivity <avi@scylladb.com>,
	Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH 05/19] Add io_uring IO interface
Date: Wed, 13 Feb 2019 00:11:41 +0100
Message-ID: <CAG48ez22G1orwZ43tZ6VK3rngQitar0qH9zJXdTv8kX2e2HqOQ@mail.gmail.com> (raw)
In-Reply-To: <beb052ba-2c4a-0201-d673-c7b8eef294dd@kernel.dk>

On Wed, Feb 13, 2019 at 12:00 AM Jens Axboe <axboe@kernel.dk> wrote:
>
> On 2/12/19 3:57 PM, Jann Horn wrote:
> > On Tue, Feb 12, 2019 at 11:52 PM Jens Axboe <axboe@kernel.dk> wrote:
> >>
> >> On 2/12/19 3:45 PM, Jens Axboe wrote:
> >>> On 2/12/19 3:40 PM, Jann Horn wrote:
> >>>> On Tue, Feb 12, 2019 at 11:06 PM Jens Axboe <axboe@kernel.dk> wrote:
> >>>>>
> >>>>> On 2/12/19 3:03 PM, Jens Axboe wrote:
> >>>>>> On 2/12/19 2:42 PM, Jann Horn wrote:
> >>>>>>> On Sat, Feb 9, 2019 at 5:15 AM Jens Axboe <axboe@kernel.dk> wrote:
> >>>>>>>> On 2/8/19 3:12 PM, Jann Horn wrote:
> >>>>>>>>> On Fri, Feb 8, 2019 at 6:34 PM Jens Axboe <axboe@kernel.dk> wrote:
> >>>>>>>>>> The submission queue (SQ) and completion queue (CQ) rings are shared
> >>>>>>>>>> between the application and the kernel. This eliminates the need to
> >>>>>>>>>> copy data back and forth to submit and complete IO.
> >>>>>>>>>>
> >>>>>>>>>> IO submissions use the io_uring_sqe data structure, and completions
> >>>>>>>>>> are generated in the form of io_uring_cqe data structures. The SQ
> >>>>>>>>>> ring is an index into the io_uring_sqe array, which makes it possible
> >>>>>>>>>> to submit a batch of IOs without them being contiguous in the ring.
> >>>>>>>>>> The CQ ring is always contiguous, as completion events are inherently
> >>>>>>>>>> unordered, and hence any io_uring_cqe entry can point back to an
> >>>>>>>>>> arbitrary submission.
> >>>>>>>>>>
> >>>>>>>>>> Two new system calls are added for this:
> >>>>>>>>>>
> >>>>>>>>>> io_uring_setup(entries, params)
> >>>>>>>>>>         Sets up an io_uring instance for doing async IO. On success,
> >>>>>>>>>>         returns a file descriptor that the application can mmap to
> >>>>>>>>>>         gain access to the SQ ring, CQ ring, and io_uring_sqes.
> >>>>>>>>>>
> >>>>>>>>>> io_uring_enter(fd, to_submit, min_complete, flags, sigset, sigsetsize)
> >>>>>>>>>>         Initiates IO against the rings mapped to this fd, or waits for
> >>>>>>>>>>         them to complete, or both. The behavior is controlled by the
> >>>>>>>>>>         parameters passed in. If 'to_submit' is non-zero, then we'll
> >>>>>>>>>>         try and submit new IO. If IORING_ENTER_GETEVENTS is set, the
> >>>>>>>>>>         kernel will wait for 'min_complete' events, if they aren't
> >>>>>>>>>>         already available. It's valid to set IORING_ENTER_GETEVENTS
> >>>>>>>>>>         and 'min_complete' == 0 at the same time, this allows the
> >>>>>>>>>>         kernel to return already completed events without waiting
> >>>>>>>>>>         for them. This is useful only for polling, as for IRQ
> >>>>>>>>>>         driven IO, the application can just check the CQ ring
> >>>>>>>>>>         without entering the kernel.
> >>>>>>>>>>
> >>>>>>>>>> With this setup, it's possible to do async IO with a single system
> >>>>>>>>>> call. Future developments will enable polled IO with this interface,
> >>>>>>>>>> and polled submission as well. The latter will enable an application
> >>>>>>>>>> to do IO without doing ANY system calls at all.
> >>>>>>>>>>
> >>>>>>>>>> For IRQ driven IO, an application only needs to enter the kernel for
> >>>>>>>>>> completions if it wants to wait for them to occur.
> >>>>>>>>>>
> >>>>>>>>>> Each io_uring is backed by a workqueue, to support buffered async IO
> >>>>>>>>>> as well. We will only punt to an async context if the command would
> >>>>>>>>>> need to wait for IO on the device side. Any data that can be accessed
> >>>>>>>>>> directly in the page cache is done inline. This avoids the slowness
> >>>>>>>>>> issue of usual threadpools, since cached data is accessed as quickly
> >>>>>>>>>> as a sync interface.
> >>>>>>> [...]
> >>>>>>>>>> +static int io_submit_sqe(struct io_ring_ctx *ctx, const struct sqe_submit *s)
> >>>>>>>>>> +{
> >>>>>>>>>> +       struct io_kiocb *req;
> >>>>>>>>>> +       ssize_t ret;
> >>>>>>>>>> +
> >>>>>>>>>> +       /* enforce forwards compatibility on users */
> >>>>>>>>>> +       if (unlikely(s->sqe->flags))
> >>>>>>>>>> +               return -EINVAL;
> >>>>>>>>>> +
> >>>>>>>>>> +       req = io_get_req(ctx);
> >>>>>>>>>> +       if (unlikely(!req))
> >>>>>>>>>> +               return -EAGAIN;
> >>>>>>>>>> +
> >>>>>>>>>> +       req->rw.ki_filp = NULL;
> >>>>>>>>>> +
> >>>>>>>>>> +       ret = __io_submit_sqe(ctx, req, s, true);
> >>>>>>>>>> +       if (ret == -EAGAIN) {
> >>>>>>>>>> +               memcpy(&req->submit, s, sizeof(*s));
> >>>>>>>>>> +               INIT_WORK(&req->work, io_sq_wq_submit_work);
> >>>>>>>>>> +               queue_work(ctx->sqo_wq, &req->work);
> >>>>>>>>>> +               ret = 0;
> >>>>>>>>>> +       }
> >>>>>>>>>> +       if (ret)
> >>>>>>>>>> +               io_free_req(req);
> >>>>>>>>>> +
> >>>>>>>>>> +       return ret;
> >>>>>>>>>> +}
> >>>>>>>>>> +
> >>>>>>>>>> +static void io_commit_sqring(struct io_ring_ctx *ctx)
> >>>>>>>>>> +{
> >>>>>>>>>> +       struct io_sq_ring *ring = ctx->sq_ring;
> >>>>>>>>>> +
> >>>>>>>>>> +       if (ctx->cached_sq_head != ring->r.head) {
> >>>>>>>>>> +               WRITE_ONCE(ring->r.head, ctx->cached_sq_head);
> >>>>>>>>>> +               /* write side barrier of head update, app has read side */
> >>>>>>>>>> +               smp_wmb();
> >>>>>>>>>
> >>>>>>>>> Can you elaborate on what this memory barrier is doing? Don't you need
> >>>>>>>>> some sort of memory barrier *before* the WRITE_ONCE(), to ensure that
> >>>>>>>>> nobody sees the updated head before you're done reading the submission
> >>>>>>>>> queue entry? Or is that barrier elsewhere?
> >>>>>>>>
> >>>>>>>> The matching read barrier is in the application, it must do that before
> >>>>>>>> reading ->head for the SQ ring.
> >>>>>>>>
> >>>>>>>> For the other barrier, since the ring->r.head now has a READ_ONCE(),
> >>>>>>>> that should be all we need to ensure that loads are done.
> >>>>>>>
> >>>>>>> READ_ONCE() / WRITE_ONCE are not hardware memory barriers that enforce
> >>>>>>> ordering with regard to concurrent execution on other cores. They are
> >>>>>>> only compiler barriers, influencing the order in which the compiler
> >>>>>>> emits things. (Well, unless you're on alpha, where READ_ONCE() implies
> >>>>>>> a memory barrier that prevents reordering of dependent reads.)
> >>>>>>>
> >>>>>>> As far as I can tell, between the READ_ONCE(ring->array[...]) in
> >>>>>>> io_get_sqring() and the WRITE_ONCE() in io_commit_sqring(), you have
> >>>>>>> no *hardware* memory barrier that prevents reordering against
> >>>>>>> concurrently running userspace code. As far as I can tell, the
> >>>>>>> following could happen:
> >>>>>>>
> >>>>>>>  - The kernel reads from ring->array in io_get_sqring(), then updates
> >>>>>>> the head in io_commit_sqring(). The CPU reorders the memory accesses
> >>>>>>> such that the write to the head becomes visible before the read from
> >>>>>>> ring->array has completed.
> >>>>>>>  - Userspace observes the write to the head and reuses the array slots
> >>>>>>> the kernel has freed with the write, clobbering ring->array before the
> >>>>>>> kernel reads from ring->array.
> >>>>>>
> >>>>>> I'd say this is highly theoretical for the normal use case, as we
> >>>>>> will have submitted IO in between. Hence the load must have been done.
> >>>>
> >>>> Sorry, I'm confused. Who is "we", and which load are you referring to?
> >>>> io_sq_thread() goes directly from io_get_sqring() to
> >>>> io_commit_sqring(), with only a conditional io_sqe_needs_user() in
> >>>> between, if the `i == ARRAY_SIZE(sqes)` check triggers. There is no
> >>>> "submitting IO" in the middle.
> >>>
> >>> You are right, the patch I sent IS needed for the sq thread case! It's
> >>> only true for the "normal" case that we don't need the smp_mb() before
> >>> writing the sq ring head, as sqes are fully consumed at that point.
> >
> > Hmm... does that actually matter? As long as you don't have an
> > explicit barrier for this, the CPU could still reorder things, right?
> > Pull the store in front of everything else?
>
> If the IO has been submitted, by definition the loads have completed.
> At that point it should be fine to commit the ring head that the
> application sees.

What exactly do you mean by "the IO has been submitted"? Are you
talking about interaction with hardware, or about the end of the
syscall, or something else?

> >>> I'll fold the fix into that patch.
> >> A better fix is to let the sq thread have the same behavior as the
> >> application driven path, simply committing the sq ring once we've
> >> consumed the sqes instead. That's just moving the io_sqring_commit()
> >> below io_submit_sqes().
> >
> > Hmm. How does that help?
>
> Because then it'll have submitted the IO, and hence loads from the sqes
> in question must have been done.

  reply index

Thread overview: 66+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-08 17:34 [PATCHSET v13] " Jens Axboe
2019-02-08 17:34 ` [PATCH 01/19] fs: add an iopoll method to struct file_operations Jens Axboe
2019-02-09  9:20   ` Hannes Reinecke
2019-02-08 17:34 ` [PATCH 02/19] block: wire up block device iopoll method Jens Axboe
2019-02-09  9:22   ` Hannes Reinecke
2019-02-08 17:34 ` [PATCH 03/19] block: add bio_set_polled() helper Jens Axboe
2019-02-09  9:24   ` Hannes Reinecke
2019-02-08 17:34 ` [PATCH 04/19] iomap: wire up the iopoll method Jens Axboe
2019-02-09  9:25   ` Hannes Reinecke
2019-02-08 17:34 ` [PATCH 05/19] Add io_uring IO interface Jens Axboe
2019-02-08 22:12   ` Jann Horn
2019-02-09  4:15     ` Jens Axboe
2019-02-12 21:42       ` Jann Horn
2019-02-12 22:03         ` Jens Axboe
2019-02-12 22:06           ` Jens Axboe
2019-02-12 22:40             ` Jann Horn
2019-02-12 22:45               ` Jens Axboe
2019-02-12 22:52                 ` Jens Axboe
2019-02-12 22:57                   ` Jann Horn
2019-02-12 23:00                     ` Jens Axboe
2019-02-12 23:11                       ` Jann Horn [this message]
2019-02-12 23:19                         ` Jens Axboe
2019-02-12 23:28                           ` Jann Horn
2019-02-12 23:46                             ` Jens Axboe
2019-02-12 23:53                               ` Jens Axboe
2019-02-13  0:07                                 ` Andy Lutomirski
2019-02-13  0:14                                   ` Jann Horn
2019-02-13  0:24                                   ` Jens Axboe
2019-02-09  9:35   ` Hannes Reinecke
2019-02-08 17:34 ` [PATCH 06/19] io_uring: add fsync support Jens Axboe
2019-02-08 22:36   ` Jann Horn
2019-02-08 23:31     ` Jens Axboe
2019-02-09  9:37   ` Hannes Reinecke
2019-02-08 17:34 ` [PATCH 07/19] io_uring: support for IO polling Jens Axboe
2019-02-09  9:39   ` Hannes Reinecke
2019-02-08 17:34 ` [PATCH 08/19] fs: add fget_many() and fput_many() Jens Axboe
2019-02-09  9:41   ` Hannes Reinecke
2019-02-08 17:34 ` [PATCH 09/19] io_uring: use fget/fput_many() for file references Jens Axboe
2019-02-09  9:42   ` Hannes Reinecke
2019-02-08 17:34 ` [PATCH 10/19] io_uring: batch io_kiocb allocation Jens Axboe
2019-02-09  9:43   ` Hannes Reinecke
2019-02-08 17:34 ` [PATCH 11/19] block: implement bio helper to add iter bvec pages to bio Jens Axboe
2019-02-09  9:45   ` Hannes Reinecke
2019-02-08 17:34 ` [PATCH 12/19] io_uring: add support for pre-mapped user IO buffers Jens Axboe
2019-02-08 22:54   ` Jann Horn
2019-02-08 23:38     ` Jens Axboe
2019-02-09 16:50       ` Jens Axboe
2019-02-09  9:48   ` Hannes Reinecke
2019-02-08 17:34 ` [PATCH 13/19] net: split out functions related to registering inflight socket files Jens Axboe
2019-02-08 19:49   ` David Miller
2019-02-08 19:51     ` Jens Axboe
2019-02-09  9:49   ` Hannes Reinecke
2019-02-08 17:34 ` [PATCH 14/19] io_uring: add file set registration Jens Axboe
2019-02-08 20:26   ` Jann Horn
2019-02-09  0:16     ` Jens Axboe
2019-02-09  9:50   ` Hannes Reinecke
2019-02-08 17:34 ` [PATCH 15/19] io_uring: add submission polling Jens Axboe
2019-02-09  9:53   ` Hannes Reinecke
2019-02-08 17:34 ` [PATCH 16/19] io_uring: add io_kiocb ref count Jens Axboe
2019-02-08 17:34 ` [PATCH 17/19] io_uring: add support for IORING_OP_POLL Jens Axboe
2019-02-08 17:34 ` [PATCH 18/19] io_uring: allow workqueue item to handle multiple buffered requests Jens Axboe
2019-02-08 17:34 ` [PATCH 19/19] io_uring: add io_uring_event cache hit information Jens Axboe
2019-02-09 21:13 [PATCHSET v14] io_uring IO interface Jens Axboe
2019-02-09 21:13 ` [PATCH 05/19] Add " Jens Axboe
2019-02-10 12:03   ` Thomas Gleixner
2019-02-10 14:19     ` Jens Axboe
2019-02-11 19:00 [PATCHSET v15] " Jens Axboe
2019-02-11 19:00 ` [PATCH 05/19] Add " Jens Axboe

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