drivers: usb: core: Don't disable irqs in usb_sg_wait() during URB submit.

usb_submit_urb() may take quite long to execute.  For example, a
single sg list may have 30 or more entries, possibly leading to that
many calls to DMA-map pages.  This can cause interrupt latency of
several hundred micro-seconds.

Avoid the problem by releasing the io->lock spinlock and re-enabling
interrupts before calling usb_submit_urb().  This opens races with
usb_sg_cancel() and sg_complete().  Handle those races by using
usb_block_urb() to stop URBs from being submitted after
usb_sg_cancel() or sg_complete() with error.

Note that usb_unlink_urb() is guaranteed to return -ENODEV if
!io->urbs[i]->dev and since the -ENODEV case is already handled,
we don't have to check for !io->urbs[i]->dev explicitly.

Before this change, reading 512MB from an ext3 filesystem on a USB
memory stick showed a throughput of 12 MB/s with about 500 missed
deadlines.

With this change, reading the same file gave the same throughput but
only one or two missed deadlines.
Signed-off-by: David Mosberger <davidm@egauge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/message.c

@@ -302,9 +302,10 @@ static void sg_complete(struct urb *urb)
 		 */
 		spin_unlock(&io->lock);
 		for (i = 0, found = 0; i < io->entries; i++) {
-			if (!io->urbs[i] || !io->urbs[i]->dev)
+			if (!io->urbs[i])
 				continue;
 			if (found) {
+				usb_block_urb(io->urbs[i]);
 				retval = usb_unlink_urb(io->urbs[i]);
 				if (retval != -EINPROGRESS &&
 				    retval != -ENODEV &&
@@ -515,12 +516,10 @@ void usb_sg_wait(struct usb_sg_request *io)
 		int retval;
 
 		io->urbs[i]->dev = io->dev;
-		retval = usb_submit_urb(io->urbs[i], GFP_ATOMIC);
-
-		/* after we submit, let completions or cancellations fire;
-		 * we handshake using io->status.
-		 */
 		spin_unlock_irq(&io->lock);
+
+		retval = usb_submit_urb(io->urbs[i], GFP_NOIO);
+
 		switch (retval) {
 			/* maybe we retrying will recover */
 		case -ENXIO:	/* hc didn't queue this one */
@@ -590,8 +589,8 @@ void usb_sg_cancel(struct usb_sg_request *io)
 		for (i = 0; i < io->entries; i++) {
 			int retval;
 
-			if (!io->urbs[i]->dev)
-				continue;
+			usb_block_urb(io->urbs[i]);
+
 			retval = usb_unlink_urb(io->urbs[i]);
 			if (retval != -EINPROGRESS
 					&& retval != -ENODEV