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>

diff --git a/drivers/usb/core/message.c b/drivers/usb/core/message.c
index 8e641b5893edb7a6195c34e43248540fbff39aa6..e13a2e5577156bde0da058192cdad6b767c720ab 100644 (file)
--- a/drivers/usb/core/message.c
+++ b/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