linux-kernel - Re: [PATCH v4 4/6] hw_random: fix unregister race.

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Date:	Tue, 25 Nov 2014 15:42:23 +0800
From:	Amos Kong <akong@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
	herbert@...dor.apana.org.au, m@...s.ch, mb@...sch.de,
	mpm@...enic.com, amit.shah@...hat.com,
	linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v4 4/6] hw_random: fix unregister race.

On Wed, Nov 12, 2014 at 02:33:00PM +1030, Rusty Russell wrote:
> Amos Kong <akong@...hat.com> writes:
> > From: Rusty Russell <rusty@...tcorp.com.au>
> >
> > 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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