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

[Jens Axboe <axboe@kernel.dk>]

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

-- 
Jens Axboe