Re: [PATCH v2] usb: raw-gadget: fix raw_event_queue_fetch locking

[Andrey Konovalov]

On Tue, Apr 7, 2020 at 4:29 PM Dan Carpenter <dan.carpenter@xxxxxxxxxx> wrote:
>
> On Tue, Apr 07, 2020 at 04:14:50PM +0200, Andrey Konovalov wrote:
> > @@ -89,11 +90,18 @@ static struct usb_raw_event *raw_event_queue_fetch(
> >        * there's at least one event queued by decrementing the semaphore,
> >        * and then take the lock to protect queue struct fields.
> >        */
> > -     if (down_interruptible(&queue->sema))
> > -             return NULL;
> > +     ret = down_interruptible(&queue->sema);
> > +     if (ret)
> > +             return ERR_PTR(ret);
> >       spin_lock_irqsave(&queue->lock, flags);
> > -     if (WARN_ON(!queue->size))
> > +     /*
> > +      * queue->size must have the same value as queue->sema counter (before
> > +      * the down_interruptible() call above), so this check is a fail-safe.
> > +      */
> > +     if (WARN_ON(!queue->size)) {
> > +             spin_unlock_irqrestore(&queue->lock, flags);
> >               return NULL;
>
> I'm sorry for not noticing this earlier.  When a function returns both
> error pointers and NULL then NULL is supposed to a special case of
> success.  For example:
>
>         my_struct_pointer = get_optional_feature();
>
> If there is a memory allocation failure then my_struct_pointer is
> -ENOMEM and we fail.  But say the optional feature is disabled, then
> we can't return a valid pointer, but it's also working as designed so
> it's not an error.  In that case we return NULL.  The surrounding code
> should be written to allow NULL pointers.
>
> So I don't think returning NULL here is correct.

No problem, sent v3, thanks!