Re: [PATCH 14/36] aio: implement IOCB_CMD_POLL

[Jeff Moyer <jmoyer@redhat.com>]

Christoph Hellwig <hch@lst.de> writes:

> Simple one-shot poll through the io_submit() interface.  To poll for
> a file descriptor the application should submit an iocb of type
> IOCB_CMD_POLL.  It will poll the fd for the events specified in the
> the first 32 bits of the aio_buf field of the iocb.
>
> Unlike poll or epoll without EPOLLONESHOT this interface always works
> in one shot mode, that is once the iocb is completed, it will have to be
> resubmitted.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>

Also acked this one in the last posting.

Acked-by: Jeff Moyer <jmoyer@redhat.com>


> ---
>  fs/aio.c                     | 102 +++++++++++++++++++++++++++++++++++++++++++
>  include/uapi/linux/aio_abi.h |   6 +--
>  2 files changed, 104 insertions(+), 4 deletions(-)
>
> diff --git a/fs/aio.c b/fs/aio.c
> index da87cbf7c67a..0bafc4975d51 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -5,6 +5,7 @@
>   *	Implements an efficient asynchronous io interface.
>   *
>   *	Copyright 2000, 2001, 2002 Red Hat, Inc.  All Rights Reserved.
> + *	Copyright 2018 Christoph Hellwig.
>   *
>   *	See ../COPYING for licensing terms.
>   */
> @@ -156,9 +157,17 @@ struct kioctx {
>  	unsigned		id;
>  };
>  
> +struct poll_iocb {
> +	struct file		*file;
> +	__poll_t		events;
> +	struct wait_queue_head	*head;
> +	struct wait_queue_entry	wait;
> +};
> +
>  struct aio_kiocb {
>  	union {
>  		struct kiocb		rw;
> +		struct poll_iocb	poll;
>  	};
>  
>  	struct kioctx		*ki_ctx;
> @@ -1565,6 +1574,96 @@ static ssize_t aio_write(struct kiocb *req, struct iocb *iocb, bool vectored,
>  	return ret;
>  }
>  
> +static void __aio_complete_poll(struct poll_iocb *req, __poll_t mask)
> +{
> +	fput(req->file);
> +	aio_complete(container_of(req, struct aio_kiocb, poll),
> +			mangle_poll(mask), 0);
> +}
> +
> +static void aio_complete_poll(struct poll_iocb *req, __poll_t mask)
> +{
> +	struct aio_kiocb *iocb = container_of(req, struct aio_kiocb, poll);
> +
> +	if (!(iocb->flags & AIO_IOCB_CANCELLED))
> +		__aio_complete_poll(req, mask);
> +}
> +
> +static int aio_poll_cancel(struct kiocb *rw)
> +{
> +	struct aio_kiocb *iocb = container_of(rw, struct aio_kiocb, rw);
> +
> +	remove_wait_queue(iocb->poll.head, &iocb->poll.wait);
> +	__aio_complete_poll(&iocb->poll, 0); /* no events to report */
> +	return 0;
> +}
> +
> +static int aio_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync,
> +		void *key)
> +{
> +	struct poll_iocb *req = container_of(wait, struct poll_iocb, wait);
> +	struct file *file = req->file;
> +	__poll_t mask = key_to_poll(key);
> +
> +	assert_spin_locked(&req->head->lock);
> +
> +	/* for instances that support it check for an event match first: */
> +	if (mask && !(mask & req->events))
> +		return 0;
> +
> +	mask = vfs_poll_mask(file, req->events);
> +	if (!mask)
> +		return 0;
> +
> +	__remove_wait_queue(req->head, &req->wait);
> +	aio_complete_poll(req, mask);
> +	return 1;
> +}
> +
> +static ssize_t aio_poll(struct aio_kiocb *aiocb, struct iocb *iocb)
> +{
> +	struct poll_iocb *req = &aiocb->poll;
> +	unsigned long flags;
> +	__poll_t mask;
> +
> +	/* reject any unknown events outside the normal event mask. */
> +	if ((u16)iocb->aio_buf != iocb->aio_buf)
> +		return -EINVAL;
> +	/* reject fields that are not defined for poll */
> +	if (iocb->aio_offset || iocb->aio_nbytes || iocb->aio_rw_flags)
> +		return -EINVAL;
> +
> +	req->events = demangle_poll(iocb->aio_buf) | POLLERR | POLLHUP;
> +	req->file = fget(iocb->aio_fildes);
> +	if (unlikely(!req->file))
> +		return -EBADF;
> +
> +	req->head = vfs_get_poll_head(req->file, req->events);
> +	if (!req->head) {
> +		fput(req->file);
> +		return -EINVAL; /* same as no support for IOCB_CMD_POLL */
> +	}
> +	if (IS_ERR(req->head)) {
> +		mask = PTR_TO_POLL(req->head);
> +		goto done;
> +	}
> +
> +	init_waitqueue_func_entry(&req->wait, aio_poll_wake);
> +
> +	spin_lock_irqsave(&req->head->lock, flags);
> +	mask = vfs_poll_mask(req->file, req->events);
> +	if (!mask) {
> +		__kiocb_set_cancel_fn(aiocb, aio_poll_cancel,
> +				AIO_IOCB_DELAYED_CANCEL);
> +		__add_wait_queue(req->head, &req->wait);
> +	}
> +	spin_unlock_irqrestore(&req->head->lock, flags);
> +done:
> +	if (mask)
> +		aio_complete_poll(req, mask);
> +	return -EIOCBQUEUED;
> +}
> +
>  static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
>  			 struct iocb *iocb, bool compat)
>  {
> @@ -1628,6 +1727,9 @@ static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
>  	case IOCB_CMD_PWRITEV:
>  		ret = aio_write(&req->rw, iocb, true, compat);
>  		break;
> +	case IOCB_CMD_POLL:
> +		ret = aio_poll(req, iocb);
> +		break;
>  	default:
>  		pr_debug("invalid aio operation %d\n", iocb->aio_lio_opcode);
>  		ret = -EINVAL;
> diff --git a/include/uapi/linux/aio_abi.h b/include/uapi/linux/aio_abi.h
> index 2c0a3415beee..ed0185945bb2 100644
> --- a/include/uapi/linux/aio_abi.h
> +++ b/include/uapi/linux/aio_abi.h
> @@ -39,10 +39,8 @@ enum {
>  	IOCB_CMD_PWRITE = 1,
>  	IOCB_CMD_FSYNC = 2,
>  	IOCB_CMD_FDSYNC = 3,
> -	/* These two are experimental.
> -	 * IOCB_CMD_PREADX = 4,
> -	 * IOCB_CMD_POLL = 5,
> -	 */
> +	/* 4 was the experimental IOCB_CMD_PREADX */
> +	IOCB_CMD_POLL = 5,
>  	IOCB_CMD_NOOP = 6,
>  	IOCB_CMD_PREADV = 7,
>  	IOCB_CMD_PWRITEV = 8,