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
> "232275a 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>

diff --git a/drivers/usb/core/sysfs.c b/drivers/usb/core/sysfs.c
index 7a620931499718f897d9b99aa9c5abbe14713d7c..c953a0f1c69513a066ceab7bb7372f946b19e8ed 100644 (file)
--- a/drivers/usb/core/sysfs.c
+++ b/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;
 }