aio: implement IOCB_CMD_POLL

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>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

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.
  */
@@ -164,10 +165,22 @@ struct fsync_iocb {
 	bool			datasync;
 };
 
+struct poll_iocb {
+	struct file		*file;
+	__poll_t		events;
+	struct wait_queue_head	*head;
+
+	union {
+		struct wait_queue_entry	wait;
+		struct work_struct	work;
+	};
+};
+
 struct aio_kiocb {
 	union {
 		struct kiocb		rw;
 		struct fsync_iocb	fsync;
+		struct poll_iocb	poll;
 	};
 
 	struct kioctx		*ki_ctx;
@@ -1558,7 +1571,6 @@ static int aio_fsync(struct fsync_iocb *req, struct iocb *iocb, bool datasync)
 	if (unlikely(iocb->aio_buf || iocb->aio_offset || iocb->aio_nbytes ||
 			iocb->aio_rw_flags))
 		return -EINVAL;
-
 	req->file = fget(iocb->aio_fildes);
 	if (unlikely(!req->file))
 		return -EBADF;
@@ -1573,6 +1585,124 @@ static int aio_fsync(struct fsync_iocb *req, struct iocb *iocb, bool datasync)
 	return -EIOCBQUEUED;
 }
 
+/* need to use list_del_init so we can check if item was present */
+static inline bool __aio_poll_remove(struct poll_iocb *req)
+{
+	if (list_empty(&req->wait.entry))
+		return false;
+	list_del_init(&req->wait.entry);
+	return true;
+}
+
+static inline void __aio_poll_complete(struct poll_iocb *req, __poll_t mask)
+{
+	struct aio_kiocb *iocb = container_of(req, struct aio_kiocb, poll);
+	struct file *file = req->file;
+
+	aio_complete(iocb, mangle_poll(mask), 0);
+	fput(file);
+}
+
+static void aio_poll_work(struct work_struct *work)
+{
+	struct poll_iocb *req = container_of(work, struct poll_iocb, work);
+
+	__aio_poll_complete(req, req->events);
+}
+
+static int aio_poll_cancel(struct kiocb *iocb)
+{
+	struct aio_kiocb *aiocb = container_of(iocb, struct aio_kiocb, rw);
+	struct poll_iocb *req = &aiocb->poll;
+	struct wait_queue_head *head = req->head;
+	bool found = false;
+
+	spin_lock(&head->lock);
+	found = __aio_poll_remove(req);
+	spin_unlock(&head->lock);
+
+	if (found) {
+		req->events = 0;
+		INIT_WORK(&req->work, aio_poll_work);
+		schedule_work(&req->work);
+	}
+	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 = file->f_op->poll_mask(file, req->events);
+	if (!mask)
+		return 0;
+
+	__aio_poll_remove(req);
+
+	req->events = mask;
+	INIT_WORK(&req->work, aio_poll_work);
+	schedule_work(&req->work);
+	return 1;
+}
+
+static ssize_t aio_poll(struct aio_kiocb *aiocb, struct iocb *iocb)
+{
+	struct kioctx *ctx = aiocb->ki_ctx;
+	struct poll_iocb *req = &aiocb->poll;
+	__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) | EPOLLERR | EPOLLHUP;
+	req->file = fget(iocb->aio_fildes);
+	if (unlikely(!req->file))
+		return -EBADF;
+	if (!file_has_poll_mask(req->file))
+		goto out_fail;
+
+	req->head = req->file->f_op->get_poll_head(req->file, req->events);
+	if (!req->head)
+		goto out_fail;
+	if (IS_ERR(req->head)) {
+		mask = EPOLLERR;
+		goto done;
+	}
+
+	init_waitqueue_func_entry(&req->wait, aio_poll_wake);
+	aiocb->ki_cancel = aio_poll_cancel;
+
+	spin_lock_irq(&ctx->ctx_lock);
+	spin_lock(&req->head->lock);
+	mask = req->file->f_op->poll_mask(req->file, req->events);
+	if (!mask) {
+		__add_wait_queue(req->head, &req->wait);
+		list_add_tail(&aiocb->ki_list, &ctx->active_reqs);
+	}
+	spin_unlock(&req->head->lock);
+	spin_unlock_irq(&ctx->ctx_lock);
+done:
+	if (mask)
+		__aio_poll_complete(req, mask);
+	return -EIOCBQUEUED;
+out_fail:
+	fput(req->file);
+	return -EINVAL; /* same as no support for IOCB_CMD_POLL */
+}
+
 static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
 			 struct iocb *iocb, bool compat)
 {
@@ -1641,6 +1771,8 @@ static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
 		break;
 	case IOCB_CMD_FDSYNC:
 		ret = aio_fsync(&req->fsync, iocb, true);
+	case IOCB_CMD_POLL:
+		ret = aio_poll(req, iocb);
 		break;
 	default:
 		pr_debug("invalid aio operation %d\n", iocb->aio_lio_opcode);

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,