On Thu, 21 Jan 2016, Oliver Neukum wrote:
> 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@xxxxxxxx>
> ---
> drivers/usb/core/sysfs.c | 6 +-----
> 1 file changed, 1 insertion(+), 5 deletions(-)
>
> diff --git a/drivers/usb/core/sysfs.c b/drivers/usb/core/sysfs.c
> index 69da1d5..1ea7244 100644
> --- a/drivers/usb/core/sysfs.c
> +++ b/drivers/usb/core/sysfs.c
> @@ -840,16 +840,13 @@ 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) {
> @@ -870,7 +867,6 @@ read_descriptors(struct file *filp, struct kobject *kobj,
> off -= srclen;
> }
> }
> - usb_unlock_device(udev);
> return count - nleft;
> }
Acked-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
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