USB: update to Documentation

this mentions a new deadlock due to advanced power management.

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/Documentation/usb/power-management.txt b/Documentation/usb/power-management.txt
index b2fc4d4a99177be7bd48dce560d1f28417c60aa9..9d31140e3f5bc2db17e446b426a4ecfacbbf99fc 100644 (file)
--- a/Documentation/usb/power-management.txt
+++ b/Documentation/usb/power-management.txt
@@ -436,7 +436,12 @@ post_reset; the USB core guarantees that this is true of internal
 suspend/resume events as well.
 
 If a driver wants to block all suspend/resume calls during some
-critical section, it can simply acquire udev->pm_mutex.
+critical section, it can simply acquire udev->pm_mutex. Note that
+calls to resume may be triggered indirectly. Block IO due to memory
+allocations can make the vm subsystem resume a device. Thus while
+holding this lock you must not allocate memory with GFP_KERNEL or
+GFP_NOFS.
+
 Alternatively, if the critical section might call some of the
 usb_autopm_* routines, the driver can avoid deadlock by doing: