[PATCH 3/8] aio_poll(): sanitize the logics after vfs_poll(), get rid of leak on error

[Al Viro <viro@ZenIV.linux.org.uk>]

From: Al Viro <viro@zeniv.linux.org.uk>

We want iocb_put() happening on errors, to balance the extra reference
we'd taken.  As it is, we end up with a leak.  The rules should be
	* error: iocb_put() to deal with the extra ref, return error,
let the caller do another iocb_put().
	* async: iocb_put() to deal with the extra ref, return 0.
	* no error, event present immediately: aio_poll_complete() to
report it, iocb_put() to deal with the extra ref, return 0.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
 fs/aio.c | 25 +++++++++++--------------
 1 file changed, 11 insertions(+), 14 deletions(-)

diff --git a/fs/aio.c b/fs/aio.c
index 3a8b894378e0..22b288997441 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -1724,6 +1724,7 @@ static ssize_t aio_poll(struct aio_kiocb *aiocb, const struct iocb *iocb)
 	struct kioctx *ctx = aiocb->ki_ctx;
 	struct poll_iocb *req = &aiocb->poll;
 	struct aio_poll_table apt;
+	bool async = false;
 	__poll_t mask;
 
 	/* reject any unknown events outside the normal event mask. */
@@ -1760,30 +1761,26 @@ static ssize_t aio_poll(struct aio_kiocb *aiocb, const struct iocb *iocb)
 
 	spin_lock_irq(&ctx->ctx_lock);
 	spin_lock(&req->head->lock);
-	if (req->woken) {
-		/* wake_up context handles the rest */
-		mask = 0;
+	if (req->woken) { /* already taken up by aio_poll_wake() */
+		async = true;
 		apt.error = 0;
-	} else if (mask || apt.error) {
-		/* if we get an error or a mask we are done */
-		WARN_ON_ONCE(list_empty(&req->wait.entry));
-		list_del_init(&req->wait.entry);
-	} else {
-		/* actually waiting for an event */
+	} else if (!mask && !apt.error) { /* actually waiting for an event */
 		list_add_tail(&aiocb->ki_list, &ctx->active_reqs);
 		aiocb->ki_cancel = aio_poll_cancel;
+		async = true;
+	} else { /* if we get an error or a mask we are done */
+		WARN_ON_ONCE(list_empty(&req->wait.entry));
+		list_del_init(&req->wait.entry);
+		/* no wakeup in the future either; aiocb is ours to dispose of */
 	}
 	spin_unlock(&req->head->lock);
 	spin_unlock_irq(&ctx->ctx_lock);
 
 out:
-	if (unlikely(apt.error))
-		return apt.error;
-
-	if (mask)
+	if (async && !apt.error)
 		aio_poll_complete(aiocb, mask);
 	iocb_put(aiocb);
-	return 0;
+	return apt.error;
 }
 
 static int __io_submit_one(struct kioctx *ctx, const struct iocb *iocb,
-- 
2.11.0