On Wed, Nov 12, 2014 at 02:33:00PM +1030, Rusty Russell wrote:
> Amos Kong <akong@xxxxxxxxxx> writes:
> > From: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
> >
> > The previous patch added one potential problem: we can still be
> > reading from a hwrng when it's unregistered. Add a wait for zero
> > in the hwrng_unregister path.
> >
> > v4: add cleanup_done flag to insure that cleanup is done
>
> That's a bit weird. The usual pattern would be to hold a reference
> until we're actually finished, but this reference is a bit weird.
The cleanup function is a callback function of kref_put(), we can't
use the same reference count inside cleanup function.
> We hold the mutex across cleanup, so we could grab that but we have to
> take care sleeping inside wait_event, otherwise Peter will have to fix
> my code again :)
We didn't hold rng_mutex inside cleanup_rng(), am I missing something?
> AFAICT the wake_woken() stuff isn't merged yet, so your patch will
> have to do for now.
Can you provide some patches/mail link here? I searched nothing about wake_woken.
> > @@ -98,6 +99,8 @@ static inline void cleanup_rng(struct kref *kref)
> >
> > if (rng->cleanup)
> > rng->cleanup(rng);
> > + rng->cleanup_done = true;
> > + wake_up_all(&rng_done);
> > }
> >
> > static void set_current_rng(struct hwrng *rng)
> > @@ -536,6 +539,11 @@ void hwrng_unregister(struct hwrng *rng)
> > kthread_stop(hwrng_fill);
> > } else
> > mutex_unlock(&rng_mutex);
> > +
> > + /* Just in case rng is reading right now, wait. */
> > + wait_event(rng_done, rng->cleanup_done &&
> > + atomic_read(&rng->ref.refcount) == 0);
> > +
>
> The atomic_read() isn't necessary here.
>
> However, you should probably init cleanup_done in hwrng_register().
> (Probably noone does unregister then register, but let's be clear).
Got it.
> Thanks,
> Rusty.
>
> > }
> > EXPORT_SYMBOL_GPL(hwrng_unregister);
> >
> > diff --git a/include/linux/hw_random.h b/include/linux/hw_random.h
> > index c212e71..7832e50 100644
> > --- a/include/linux/hw_random.h
> > +++ b/include/linux/hw_random.h
> > @@ -46,6 +46,7 @@ struct hwrng {
> > /* internal. */
> > struct list_head list;
> > struct kref ref;
> > + bool cleanup_done;
> > };
> >
> > /** Register a new Hardware Random Number Generator driver. */
> > --
> > 1.9.3
--
Amos.
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