linux-kernel - Re: [PATCH 4/8] io_uring: add support for futex wake and wait

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Message-ID: <20230927090501.GB21810@noisy.programming.kicks-ass.net>
Date:   Wed, 27 Sep 2023 11:05:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     io-uring@...r.kernel.org, linux-kernel@...r.kernel.org,
        andres@...razel.de, tglx@...utronix.de
Subject: Re: [PATCH 4/8] io_uring: add support for futex wake and wait

On Thu, Sep 21, 2023 at 12:29:04PM -0600, Jens Axboe wrote:

> +int io_futex_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
> +{
> +	struct io_futex *iof = io_kiocb_to_cmd(req, struct io_futex);
> +	u32 flags;
> +
> +	if (unlikely(sqe->fd || sqe->len || sqe->buf_index || sqe->file_index))
> +		return -EINVAL;
> +
> +	iof->uaddr = u64_to_user_ptr(READ_ONCE(sqe->addr));
> +	iof->futex_val = READ_ONCE(sqe->addr2);
> +	iof->futex_mask = READ_ONCE(sqe->addr3);
> +	flags = READ_ONCE(sqe->futex_flags);
> +
> +	if (flags & ~FUTEX2_VALID_MASK)
> +		return -EINVAL;
> +
> +	iof->futex_flags = futex2_to_flags(flags);

So prep does the flags conversion..

> +	if (!futex_flags_valid(iof->futex_flags))
> +		return -EINVAL;
> +
> +	if (!futex_validate_input(iof->futex_flags, iof->futex_val) ||
> +	    !futex_validate_input(iof->futex_flags, iof->futex_mask))
> +		return -EINVAL;
> +
> +	return 0;
> +}

> +int io_futex_wait(struct io_kiocb *req, unsigned int issue_flags)
> +{
> +	struct io_futex *iof = io_kiocb_to_cmd(req, struct io_futex);
> +	struct io_ring_ctx *ctx = req->ctx;
> +	struct io_futex_data *ifd = NULL;
> +	struct futex_hash_bucket *hb;
> +	int ret;
> +
> +	if (!iof->futex_mask) {
> +		ret = -EINVAL;
> +		goto done;
> +	}
> +
> +	io_ring_submit_lock(ctx, issue_flags);
> +	ifd = io_alloc_ifd(ctx);
> +	if (!ifd) {
> +		ret = -ENOMEM;
> +		goto done_unlock;
> +	}
> +
> +	req->async_data = ifd;
> +	ifd->q = futex_q_init;
> +	ifd->q.bitset = iof->futex_mask;
> +	ifd->q.wake = io_futex_wake_fn;
> +	ifd->req = req;
> +
> +	ret = futex_wait_setup(iof->uaddr, iof->futex_val,
> +			       futex2_to_flags(iof->futex_flags), &ifd->q, &hb);

But then wait and..

> +	if (!ret) {
> +		hlist_add_head(&req->hash_node, &ctx->futex_list);
> +		io_ring_submit_unlock(ctx, issue_flags);
> +
> +		futex_queue(&ifd->q, hb);
> +		return IOU_ISSUE_SKIP_COMPLETE;
> +	}
> +
> +done_unlock:
> +	io_ring_submit_unlock(ctx, issue_flags);
> +done:
> +	if (ret < 0)
> +		req_set_fail(req);
> +	io_req_set_res(req, ret, 0);
> +	kfree(ifd);
> +	return IOU_OK;
> +}
> +
> +int io_futex_wake(struct io_kiocb *req, unsigned int issue_flags)
> +{
> +	struct io_futex *iof = io_kiocb_to_cmd(req, struct io_futex);
> +	int ret;
> +
> +	ret = futex_wake(iof->uaddr, futex2_to_flags(iof->futex_flags),

... wake do it both again?

Also, I think we want wake to have wake do: 

  'FLAGS_STRICT | iof->futex_flags'

See 43adf8449510 ("futex: FLAGS_STRICT"), I'm thinking that waking 0
futexes should honour that request by waking 0, not 1 :-)

> +			 iof->futex_val, iof->futex_mask);
> +	if (ret < 0)
> +		req_set_fail(req);
> +	io_req_set_res(req, ret, 0);
> +	return IOU_OK;
> +}