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

From: Amos Kong <akong@redhat.com>
To: Rusty Russell <rusty@rustcorp.com.au>
Cc: herbert@gondor.apana.org.au, kvm@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org, m@bues.ch,
	mpm@selenic.com, amit.shah@redhat.com
Subject: Re: [PATCH v4 4/6] hw_random: fix unregister race.
Date: Sat, 6 Dec 2014 11:51:16 +0800	[thread overview]
Message-ID: <20141206035116.GA1995@air.redhat.com> (raw)
In-Reply-To: <87y4rhm5fv.fsf@rustcorp.com.au>

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On Wed, Nov 12, 2014 at 02:33:00PM +1030, Rusty Russell wrote:
> Amos Kong <akong@redhat.com> writes:
> > From: Rusty Russell <rusty@rustcorp.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.
> 
> 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 :)
> 
> AFAICT the wake_woken() stuff isn't merged yet, so your patch will
> have to do for now.
> 
> > @@ -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.

At least, we need it to convert refcount from atomic_t to int.
Otherwise, we will touch this error:

  error: invalid operands to binary == (have 'atomic_t' and 'int')

> However, you should probably init cleanup_done in hwrng_register().
> (Probably noone does unregister then register, but let's be clear).
> 
> Thanks,
> Rusty.

-- 
			Amos.

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