[PATCH] Add ENOSYS into sys_io_cancel

[Shiow-wen Cheng <cheng_27513@yahoo.com>]

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Other than few exceptions (e.g. usb/gadget), none of
the current filesystems and/or drivers that has
io_cancel implemented (kiocb->ki_cancel left with
NULL). However, the io_cancel() system call
(sys_io_cancel) somehow universally sets return code
to -EAGAIN. This gives us a false impression that
io_cancel() is supported (implemented) but just never
works - would appreciate if this patch is taken.

-- s.w.

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--- linux-2.6.12/fs/aio.c	2005-06-17 15:48:29.000000000 -0400
+++ linux/fs/aio.c	2005-07-10 12:48:14.000000000 -0400
@@ -1641,8 +1641,9 @@ asmlinkage long sys_io_cancel(aio_contex
 		cancel = kiocb->ki_cancel;
 		kiocb->ki_users ++;
 		kiocbSetCancelled(kiocb);
-	} else
+	} else 
 		cancel = NULL;
+	 
 	spin_unlock_irq(&ctx->ctx_lock);
 
 	if (NULL != cancel) {
@@ -1659,8 +1660,10 @@ asmlinkage long sys_io_cancel(aio_contex
 			if (copy_to_user(result, &tmp, sizeof(tmp)))
 				ret = -EFAULT;
 		}
-	} else
+	} else {
+		ret = -ENOSYS;
 		printk(KERN_DEBUG "iocb has no cancel operation\n");
+	} 
 
 	put_ioctx(ctx);