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From:   Christoph Hellwig <hch@lst.de>
To:     viro@zeniv.linux.org.uk
Cc:     Avi Kivity <avi@scylladb.com>, linux-aio@kvack.org,
        linux-fsdevel@vger.kernel.org, netdev@vger.kernel.org,
        linux-api@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH 05/36] aio: simplify cancellation
Date:   Wed, 17 Jan 2018 20:27:11 +0100
Message-Id: <20180117192742.710-6-hch@lst.de>
In-Reply-To: <20180117192742.710-1-hch@lst.de>
References: <20180117192742.710-1-hch@lst.de>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

With the current aio code there is no need for the magic KIOCB_CANCELLED
value, as a cancelation just kicks the driver to queue the completion
ASAP, with all actual completion handling done in another thread. Given
that both the completion path and cancelation take the context lock there
is no need for magic cmpxchg loops either.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jeff Moyer <jmoyer@redhat.com>
---
 fs/aio.c | 37 +++++++++----------------------------
 1 file changed, 9 insertions(+), 28 deletions(-)

diff --git a/fs/aio.c b/fs/aio.c
index c32c315f05b5..2d40cf5dd4ec 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -156,19 +156,6 @@ struct kioctx {
 	unsigned		id;
 };
 
-/*
- * We use ki_cancel == KIOCB_CANCELLED to indicate that a kiocb has been either
- * cancelled or completed (this makes a certain amount of sense because
- * successful cancellation - io_cancel() - does deliver the completion to
- * userspace).
- *
- * And since most things don't implement kiocb cancellation and we'd really like
- * kiocb completion to be lockless when possible, we use ki_cancel to
- * synchronize cancellation and completion - we only set it to KIOCB_CANCELLED
- * with xchg() or cmpxchg(), see batch_complete_aio() and kiocb_cancel().
- */
-#define KIOCB_CANCELLED		((void *) (~0ULL))
-
 struct aio_kiocb {
 	union {
 		struct kiocb		rw;
@@ -565,24 +552,18 @@ void kiocb_set_cancel_fn(struct kiocb *iocb, kiocb_cancel_fn *cancel)
 }
 EXPORT_SYMBOL(kiocb_set_cancel_fn);
 
+/*
+ * Only cancel if there ws a ki_cancel function to start with, and we
+ * are the one how managed to clear it (to protect against simulatinious
+ * cancel calls).
+ */
 static int kiocb_cancel(struct aio_kiocb *kiocb)
 {
-	kiocb_cancel_fn *old, *cancel;
-
-	/*
-	 * Don't want to set kiocb->ki_cancel = KIOCB_CANCELLED unless it
-	 * actually has a cancel function, hence the cmpxchg()
-	 */
-
-	cancel = READ_ONCE(kiocb->ki_cancel);
-	do {
-		if (!cancel || cancel == KIOCB_CANCELLED)
-			return -EINVAL;
-
-		old = cancel;
-		cancel = cmpxchg(&kiocb->ki_cancel, old, KIOCB_CANCELLED);
-	} while (cancel != old);
+	kiocb_cancel_fn *cancel = kiocb->ki_cancel;
 
+	if (!cancel)
+		return -EINVAL;
+	kiocb->ki_cancel = NULL;
 	return cancel(&kiocb->rw);
 }
 
-- 
2.14.2