usb: no locking for reading descriptors in sysfs

Quting the relevant thread:

> In fact, I suspect the locking added by the kernel 3.13 commit for
> read_descriptors() is invalid because read_descriptors() performs no USB
> activity; read_descriptors() just reads information from an allocated
> memory structure. This structure is protected as the structure is
> existing before and after the sysfs vfs descriptors entry is created or
> destroyed.

You're right.  For some reason I thought that usb_deauthorize_device()
would destroy the rawdescriptor structures (as mentioned in that
commit's Changelog), but it doesn't.  The locking in read_descriptors()
is unnecessary.

> The information is only written at the time of enumeration
> and does not change. At least that is my understanding.
>
> It is noted that in our testing of kernel 3.8 on ARM, that sysfs
> read_descriptors() was non-blocking because the kernel 3.13 comment was
> not there.
>
> The pre-kernel 3.13 sysfs read_descriptors() seemed to work OK.
>
> Proposal:
> =========
>
> Remove the usb_lock_device(udev) and usb_unlock_device(udev) from
> devices/usb/core/sysfs.c in read_descriptors() that was added by the
> kernel 3.13 commit
> "232275a0 USB: fix substandard locking for the sysfs files"
>
> Any comments to this proposal ?

It seems okay to me.  Please submit a patch.

So this removes the locking making the point about -EINTR in
the first path moot.
Signed-off-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/sysfs.c

@@ -843,16 +843,13 @@ read_descriptors(struct file *filp, struct kobject *kobj,
 	struct usb_device *udev = to_usb_device(dev);
 	size_t nleft = count;
 	size_t srclen, n;
-	int cfgno, rc;
+	int cfgno;
 	void *src;
 
 	/* The binary attribute begins with the device descriptor.
 	 * Following that are the raw descriptor entries for all the
 	 * configurations (config plus subsidiary descriptors).
 	 */
-	rc = usb_lock_device_interruptible(udev);
-	if (rc < 0)
-		return -EINTR;
 	for (cfgno = -1; cfgno < udev->descriptor.bNumConfigurations &&
 			nleft > 0; ++cfgno) {
 		if (cfgno < 0) {
@@ -873,7 +870,6 @@ read_descriptors(struct file *filp, struct kobject *kobj,
 			off -= srclen;
 		}
 	}
-	usb_unlock_device(udev);
 	return count - nleft;
 }