Re: [bug report] usb: gadget: add raw-gadget interface

[Dan Carpenter <dan.carpenter@oracle.com>]

On Mon, Apr 06, 2020 at 03:06:12PM +0200, Andrey Konovalov wrote:
> On Mon, Apr 6, 2020 at 12:55 PM Dan Carpenter <dan.carpenter@oracle.com> wrote:
> >
> > Hello Andrey Konovalov,
> >
> > The patch f2c2e717642c: "usb: gadget: add raw-gadget interface" from
> > Feb 24, 2020, leads to the following static checker warning:
> >
> >     drivers/usb/gadget/legacy/raw_gadget.c:102 raw_event_queue_fetch() warn: inconsistent returns 'queue->sema'.
> >       Locked on  : 96,102
> >       Unlocked on: 93
> >
> > drivers/usb/gadget/legacy/raw_gadget.c
> >     81  static struct usb_raw_event *raw_event_queue_fetch(
> >     82                                  struct raw_event_queue *queue)
> >     83  {
> >     84          unsigned long flags;
> >     85          struct usb_raw_event *event;
> >     86
> >     87          /*
> >     88           * This function can be called concurrently. We first check that
> >     89           * there's at least one event queued by decrementing the semaphore,
> >     90           * and then take the lock to protect queue struct fields.
> >     91           */
> >     92          if (down_interruptible(&queue->sema))
> >     93                  return NULL;
> >     94          spin_lock_irqsave(&queue->lock, flags);
> >     95          if (WARN_ON(!queue->size))
> >     96                  return NULL;
> >
> > I'm going to have investigate to see why Smatch doesn't complain that
> > we have disabled IRQs on this path...  Anyway, this doesn't seem like it
> > can be correct.  I get that this is a WARN_ON() path, but we're leaving
> > the computer in a very screwed up state we don't at least enable IRQs.
> 
> Hi Dan,
> 
> Oh, right, I'll send a patch to add spin_lock_irqsave() there.
> 
> I don't really get the warning about queue->sema though, is there
> something wrong with it, or is it actually a warning about
> queue->lock?

The rule here is that we return NULL here and on line 93, so how does
the caller know if we took that "queue->sema" lock?

regards,
dan carpenter