[AIO] implement threads based aio fallback path

In order to make AIO more generally useful to userland applications, 
provide a thread based fallback for AIO operations that do not implement 
the AIO versions of file_operations.

Signed-off-by: Benjamin LaHaise <bcrl@linux.intel.com>
diff -purN --exclude=description 818_aio_vectored/fs/aio.c 82_aio_threads/fs/aio.c
--- 818_aio_vectored/fs/aio.c	2005-08-17 13:46:51.000000000 -0400
+++ 82_aio_threads/fs/aio.c	2005-08-17 13:46:54.000000000 -0400
@@ -1507,6 +1507,92 @@ ssize_t aio_pwrite(struct kiocb *iocb)
 	return ret;
 }
 
+static int aio_thread_read_fn(void *arg)
+{
+	struct kiocb *iocb = arg;
+	long ret;
+
+	set_fs(USER_DS);
+	use_mm(iocb->ki_ctx->mm);
+	ret = iocb->ki_filp->f_op->read(iocb->ki_filp, iocb->ki_buf, 
+					iocb->ki_left, &iocb->ki_pos);
+	unuse_mm(iocb->ki_ctx->mm);
+	aio_complete(iocb, ret, 0);
+
+	return 0;
+}
+
+static int aio_thread_write_fn(void *arg)
+{
+	struct kiocb *iocb = arg;
+	long ret;
+
+	set_fs(USER_DS);
+	use_mm(iocb->ki_ctx->mm);
+	ret = iocb->ki_filp->f_op->write(iocb->ki_filp, iocb->ki_buf, 
+					 iocb->ki_left, &iocb->ki_pos);
+	unuse_mm(iocb->ki_ctx->mm);
+	aio_complete(iocb, ret, 0);
+
+	return 0;
+}
+
+static int aio_thread_fsync_fn(void *arg)
+{
+	struct kiocb *iocb = arg;
+	long ret;
+
+	ret = iocb->ki_filp->f_op->fsync(iocb->ki_filp,
+					 iocb->ki_filp->f_dentry, 0);
+	aio_complete(iocb, ret, 0);
+
+	return 0;
+}
+
+static int aio_thread_fdsync_fn(void *arg)
+{
+	struct kiocb *iocb = arg;
+	long ret;
+
+	ret = iocb->ki_filp->f_op->fsync(iocb->ki_filp,
+					 iocb->ki_filp->f_dentry, 1);
+	aio_complete(iocb, ret, 0);
+
+	return 0;
+}
+
+static ssize_t aio_kernel_thread(struct kiocb *iocb, int (*fn)(void *))
+{
+	long ret;
+	/* We don't use CLONE_VM as doing so means exit_aio() will not get 
+	 * called when the aio user exits.
+	 */
+	ret = kernel_thread(fn, iocb, CLONE_FS|CLONE_FILES);
+	if (ret < 0)
+		return ret;
+	return -EIOCBQUEUED;
+}
+
+static ssize_t aio_thread_pread(struct kiocb *iocb)
+{
+	return aio_kernel_thread(iocb, aio_thread_read_fn);
+}
+
+static ssize_t aio_thread_pwrite(struct kiocb *iocb)
+{
+	return aio_kernel_thread(iocb, aio_thread_write_fn);
+}
+
+static ssize_t aio_thread_fsync(struct kiocb *iocb)
+{
+	return aio_kernel_thread(iocb, aio_thread_fsync_fn);
+}
+
+static ssize_t aio_thread_fdsync(struct kiocb *iocb)
+{
+	return aio_kernel_thread(iocb, aio_thread_fdsync_fn);
+}
+
 static ssize_t aio_fdsync(struct kiocb *iocb)
 {
 	struct file *file = iocb->ki_filp;
@@ -1570,6 +1656,8 @@ static ssize_t aio_setup_iocb(struct kio
 		ret = -EINVAL;
 		if (file->f_op->aio_read)
 			kiocb->ki_retry = aio_pread;
+		else if (file->f_op->read)
+			kiocb->ki_retry = aio_thread_pread;
 		break;
 	case IOCB_CMD_PWRITE:
 		ret = -EBADF;
@@ -1582,6 +1670,8 @@ static ssize_t aio_setup_iocb(struct kio
 		ret = -EINVAL;
 		if (file->f_op->aio_write)
 			kiocb->ki_retry = aio_pwrite;
+		else if (file->f_op->write)
+			kiocb->ki_retry = aio_thread_pwrite;
 		break;
 	case IOCB_CMD_PREADV:
 		ret = -EBADF;
@@ -1609,11 +1699,15 @@ static ssize_t aio_setup_iocb(struct kio
 		ret = -EINVAL;
 		if (file->f_op->aio_fsync)
 			kiocb->ki_retry = aio_fdsync;
+		else if (file->f_op->fsync)
+			kiocb->ki_retry = aio_thread_fdsync;
 		break;
 	case IOCB_CMD_FSYNC:
 		ret = -EINVAL;
 		if (file->f_op->aio_fsync)
 			kiocb->ki_retry = aio_fsync;
+		else if (file->f_op->fsync)
+			kiocb->ki_retry = aio_thread_fsync;
 		break;
 	default:
 		dprintk("EINVAL: io_submit: no operation provided\n");