USB: EHCI: don't lose events during a scan

This patch (as1584) fixes a minor bug that has been present in
ehci-hcd since the beginning.

Scanning the schedules for URB completions is single-threaded.  If a
completion interrupt occurs while an URB is being given back, the
interrupt handler realizes that a scan is in progress on another CPU
and avoids starting a new one.

This means that completion events can be lost.  If an URB completes
after it has been scanned but while a scan is still in progress, the
driver won't notice and won't rescan the completed URB.

The patch fixes the problem by adding a new flag to indicate that
another scan is needed after the current scan is done.  The flag gets
set whenever a completion interrupt occurs while a scan is in
progress.  The rescan will see the completion, thus preventing it from
getting lost.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 12 insertions, 4 deletions

diff --git a/drivers/usb/host/ehci-hcd.c b/drivers/usb/host/ehci-hcd.c
index 74ffd20edff8..f6cf1d178107 100644
--- a/drivers/usb/host/ehci-hcd.c
+++ b/drivers/usb/host/ehci-hcd.c
@@ -400,14 +400,21 @@ static void ehci_work (struct ehci_hcd *ehci)
          * it reports urb completions.  this flag guards against bogus
          * attempts at re-entrant schedule scanning.
          */
-        if (ehci->scanning)
+        if (ehci->scanning) {
+                ehci->need_rescan = true;
                 return;
-        ehci->scanning = 1;
+        }
+        ehci->scanning = true;
+
+ rescan:
+        ehci->need_rescan = false;
         if (ehci->async_count)
                 scan_async(ehci);
         if (ehci->next_uframe != -1)
                 scan_periodic (ehci);
-        ehci->scanning = 0;
+        if (ehci->need_rescan)
+                goto rescan;
+        ehci->scanning = false;
 
         /* the IO watchdog guards against hardware or driver bugs that
          * misplace IRQs, and should let us run completely without IRQs.
diff --git a/drivers/usb/host/ehci.h b/drivers/usb/host/ehci.h
index 303c36cc99c9..c462d52ac575 100644
--- a/drivers/usb/host/ehci.h
+++ b/drivers/usb/host/ehci.h
@@ -113,7 +113,8 @@ struct ehci_hcd {			/* one per controller */
         enum ehci_rh_state      rh_state;
 
         /* general schedule support */
-        unsigned                scanning:1;
+        bool                    scanning:1;
+        bool                    need_rescan:1;
         bool                    intr_unlinking:1;
         bool                    async_unlinking:1;