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-rw-r--r--Documentation/usb/power-management.txt7
1 files changed, 6 insertions, 1 deletions
diff --git a/Documentation/usb/power-management.txt b/Documentation/usb/power-management.txt
index b2fc4d4a991..9d31140e3f5 100644
--- 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: