From: Pavel Begunkov <asml.silence@gmail.com>
To: Jens Axboe <axboe@kernel.dk>, io-uring@vger.kernel.org
Cc: andres@anarazel.de
Subject: Re: [PATCH 2/6] io_uring: add IORING_OP_PROVIDE_BUFFERS
Date: Sat, 29 Feb 2020 14:36:14 +0300 [thread overview]
Message-ID: <d7eebc93-4efb-730c-21fd-d866dd54eaa6@gmail.com> (raw)
In-Reply-To: <297b6e91-b683-80a3-38a2-9c3b845c4d06@kernel.dk>
On 2/29/2020 7:50 AM, Jens Axboe wrote:
> On 2/28/20 5:43 PM, Pavel Begunkov wrote:
>>> +static int io_provide_buffers(struct io_kiocb *req, struct io_kiocb **nxt,
>>> + bool force_nonblock)
>>> +{
>>> + struct io_provide_buf *p = &req->pbuf;
>>> + struct io_ring_ctx *ctx = req->ctx;
>>> + struct list_head *list;
>>> + int ret = 0;
>>> +
>>> + /*
>>> + * "Normal" inline submissions always hold the uring_lock, since we
>>> + * grab it from the system call. Same is true for the SQPOLL offload.
>>> + * The only exception is when we've detached the request and issue it
>>> + * from an async worker thread, grab the lock for that case.
>>> + */
>>> + if (!force_nonblock)
>>> + mutex_lock(&ctx->uring_lock);
>>
>> io_poll_task_handler() calls it with force_nonblock==true, but it
>> doesn't hold the mutex AFAIK.
>
> True, that's the only exception. And that command doesn't transfer data
> so would never need a buffer, but I agree that's perhaps not fully
> clear. The async task handler grabs the mutex.
Hmm, I meant io_poll_task_func(), which do __io_queue_sqe() for @nxt
request, which in its turn calls io_issue_sqe(force_nonblock=true).
Does io_poll_task_func() hold @uring_mutex? Otherwise, if @nxt happened
to be io_provide_buffers(), we get there without holding the mutex and
with force_nonblock=true.
>>> + lockdep_assert_held(&ctx->uring_lock);
>>> +
>>> + list = idr_find(&ctx->io_buffer_idr, p->gid);
>>> + if (!list) {
>>> + list = kmalloc(sizeof(*list), GFP_KERNEL);
>>> + if (!list) {
>>> + ret = -ENOMEM;
>>> + goto out;
>>> + }
>>> + INIT_LIST_HEAD(list);
>>> + ret = idr_alloc(&ctx->io_buffer_idr, list, p->gid, p->gid + 1,
>>> + GFP_KERNEL);
>>> + if (ret < 0) {
>>> + kfree(list);
>>> + goto out;
>>> + }
>>> + }
>>> +
>>> + ret = io_add_buffers(p, list);
>>
>> Isn't it better to not do partial registration?
>> i.e. it may return ret < pbuf->nbufs
>
> Most things work like that, though. If you ask for an 8k read, you can't
> unwind if you just get 4k. We return 4k for that. I think in general, if
> it fails, you're probably somewhat screwed in either case. At least with
> the partial return, you know which buffers got registered and how many
> you can use. If you return 0 and undo it all, then the application
> really has no way to continue except abort.
>
--
Pavel Begunkov
next prev parent reply other threads:[~2020-02-29 11:36 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-28 20:30 [PATCHSET v3] io_uring support for automatic buffers Jens Axboe
2020-02-28 20:30 ` [PATCH 1/6] io_uring: buffer registration infrastructure Jens Axboe
2020-02-28 20:30 ` [PATCH 2/6] io_uring: add IORING_OP_PROVIDE_BUFFERS Jens Axboe
2020-02-29 0:43 ` Pavel Begunkov
2020-02-29 4:50 ` Jens Axboe
2020-02-29 11:36 ` Pavel Begunkov [this message]
2020-02-29 17:32 ` Jens Axboe
2020-02-29 12:08 ` Pavel Begunkov
2020-02-29 17:34 ` Jens Axboe
2020-02-29 18:11 ` Jens Axboe
2020-03-09 17:03 ` Andres Freund
2020-03-09 17:17 ` Jens Axboe
2020-03-09 17:28 ` Andres Freund
2020-03-10 13:33 ` Jens Axboe
2020-02-28 20:30 ` [PATCH 3/6] io_uring: support buffer selection Jens Axboe
2020-02-29 12:21 ` Pavel Begunkov
2020-02-29 17:35 ` Jens Axboe
2020-03-09 17:21 ` Andres Freund
2020-03-10 13:37 ` Jens Axboe
2020-02-28 20:30 ` [PATCH 4/6] io_uring: add IOSQE_BUFFER_SELECT support for IORING_OP_READV Jens Axboe
2020-02-28 20:30 ` [PATCH 5/6] net: abstract out normal and compat msghdr import Jens Axboe
2020-02-28 20:30 ` [PATCH 6/6] io_uring: add IOSQE_BUFFER_SELECT support for IORING_OP_RECVMSG Jens Axboe
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