Re: [PATCH v4] uart: fix race between uart_put_char() and uart_shutdown()

[Tycho Andersen <tycho@tycho.ws>]

Hi Andy,

On Thu, Jul 12, 2018 at 12:03:08PM +0300, Andy Shevchenko wrote:
> On Wed, Jul 11, 2018 at 7:07 PM, Tycho Andersen <tycho@tycho.ws> wrote:
> 
> > Anyway, since the lock is not acquired, if uart_shutdown() is called, the
> > last chunk of that function may release state->xmit.buf before its assigned
> > to null, and cause the race above.
> >
> > To fix it, let's lock uport->lock when allocating/deallocating
> > state->xmit.buf in addition to the per-port mutex.
> 
> >     * use the result of uart_port_lock() in uart_shutdown() to avoid
> >       uninitialized warning
> >     * don't use the uart_port_lock/unlock macros in uart_port_startup,
> >       instead test against uport directly; the compiler can't seem to "see"
> >       through the macros/ref/unref calls to not warn about uninitialized
> >       flags. We don't need to do a ref here since we hold the per-port
> >       mutex anyway.
> 
> > +       if (uport)
> > +               spin_lock_irqsave(&uport->lock, flags);
> 
> > +       if (uport)
> > +               spin_unlock_irqrestore(&uport->lock, flags);
> 
> At some point it It was uart_port_lock()/uart_port_unlock(), and you
> changed to simple spin lock. The macro also take reference to the
> port. Do we aware about that here?

I don't think so, the commit message you quoted above says,

> We don't need to do a ref here since we hold the per-port mutex
> anyway.

Cheers,

Tycho