Re: [PATCH 05/19] Add io_uring IO interface

[Andy Lutomirski <luto@amacapital.net>]

> On Feb 12, 2019, at 3:53 PM, Jens Axboe <axboe@kernel.dk> wrote:
> 
>> On 2/12/19 4:46 PM, Jens Axboe wrote:
>>> On 2/12/19 4:28 PM, Jann Horn wrote:
>>>> On Wed, Feb 13, 2019 at 12:19 AM Jens Axboe <axboe@kernel.dk> wrote:
>>>> 
>>>>> On 2/12/19 4:11 PM, Jann Horn wrote:
>>>>>> 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 mean that the loads from the sqe, which the IO is made of, have been
>>>> done. That's what we care about here, right? The sqe has either been
>>>> turned into an io request and has been submitted, or it has been copied.
>>> 
>>> But they might not actually be done. AFAIU the CPU is allowed to do
>>> the WRITE_ONCE of the head before doing any of the reads from the sqe
>>> - loads and stores you do, as observed by a concurrently executing
>>> thread, can happen in an order independent of the order in which you
>>> write them in your code unless you use memory barriers. So the CPU
>>> might decide to first write the new head, then do the read for
>>> io_get_sqring(), and then do the __io_submit_sqe(), potentially
>>> reading e.g. a IORING_OP_NOP opcode that has been written by
>>> concurrently executing userspace after userspace has observed the
>>> bumped head.
>> 
>> For that to be possible, we'd need NO ordering in between the IO
>> submission and when we write the sq ring head. A single spin lock
>> should do it, right?
>> 
>> It's not that I'm set against adding an smp_mb() to io_commit_sqring(),
>> but I think we're going off the deep end a little bit here on
>> theoretical vs what can practically happen.
>> 
>> For the regular IO cases, we will have done at least one lock/unlock
>> cycle. This is true for nops as well, and poll. The only case that could
>> potentially NOT have one is the fsync, for the case where we punt and
>> don't add it to existing work, we don't have any locking in between.
>> 
>> I'll add the smp_mb() for peace of mind.
> 
> For reference, folded in:
> 
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 8d68569f9ba9..755ff8f411da 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -1690,6 +1690,13 @@ static void io_commit_sqring(struct io_ring_ctx *ctx)
>    struct io_sq_ring *ring = ctx->sq_ring;
> 
>    if (ctx->cached_sq_head != READ_ONCE(ring->r.head)) {
> +        /*
> +         * Ensure any loads from the SQEs are done at this point,
> +         * since once we write the new head, the application could
> +         * write new data to them.
> +         */
> +        smp_mb();
> +
>        WRITE_ONCE(ring->r.head, ctx->cached_sq_head);
>        /*
>         * write side barrier of head update, app has read side. See
> 
> 

I haven’t followed the full set of machinations here, but would smp_store_release() be sufficient?  It is a *lot* faster on some architectures.