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

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Message-ID: <87y4rhm5fv.fsf@rustcorp.com.au>
Date:	Wed, 12 Nov 2014 14:33:00 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Amos Kong <akong@...hat.com>,
	virtualization@...ts.linux-foundation.org
Cc:	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.

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.

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.

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

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
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