[PATCH] aio: fix kiocb_cancel() funcition

[Robert Baldyga]

This patch solves problem related with lack of possibility to call
aio_complete() from cancel callback. It was because it needs to lock
ctx->ctx_lock which is always locked before kiocb_cancel() call. So there
was no way to complete request properly.

Now spinlock is unlocked before cancel callback function call, so spinlock
recursion will not occur in case of aio_complete() call. After cancel function
call spinlock is locked back.

There is also __must_hold() macro added for kiocb_cancel() function, to allow
sparse spin lock checking.

Signed-off-by: Robert Baldyga <r.baldyga@xxxxxxxxxxx>
---

This patch was created according to suggestions of Benjamin LaHaise
(https://lkml.org/lkml/2014/1/23/336).

 fs/aio.c |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/fs/aio.c b/fs/aio.c
index 062a5f6..6d5cd9e 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -472,8 +472,10 @@ void kiocb_set_cancel_fn(struct kiocb *req, kiocb_cancel_fn *cancel)
 EXPORT_SYMBOL(kiocb_set_cancel_fn);
 
 static int kiocb_cancel(struct kioctx *ctx, struct kiocb *kiocb)
+			__must_hold(&ctx->ctx_lock)
 {
 	kiocb_cancel_fn *old, *cancel;
+	int ret;
 
 	/*
 	 * Don't want to set kiocb->ki_cancel = KIOCB_CANCELLED unless it
@@ -489,7 +491,15 @@ static int kiocb_cancel(struct kioctx *ctx, struct kiocb *kiocb)
 		cancel = cmpxchg(&kiocb->ki_cancel, old, KIOCB_CANCELLED);
 	} while (cancel != old);
 
-	return cancel(kiocb);
+	/*
+	 * cancel() function may call aio_complete function, which needs to
+	 * lock ctx->ctx_lock, so we call cancel() with spinlock unlocked
+	 */
+	spin_unlock(&ctx->ctx_lock);
+	ret = cancel(kiocb);
+	spin_lock(&ctx->ctx_lock);
+
+	return ret;
 }
 
 static void free_ioctx(struct work_struct *work)
-- 
1.7.9.5

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