Re: [PATCH v4 3/6] hw_random: use reference counts on each struct hwrng.

From: Amos Kong <akong@redhat.com>
To: Rusty Russell <rusty@rustcorp.com.au>
Cc: m@bues.ch, kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org, mb@bu3sch.de,
	mpm@selenic.com, amit.shah@redhat.com,
	herbert@gondor.apana.org.au
Subject: Re: [PATCH v4 3/6] hw_random: use reference counts on each struct hwrng.
Date: Mon, 17 Nov 2014 23:20:53 +0800	[thread overview]
Message-ID: <20141117152053.GB12501@air.redhat.com> (raw)
In-Reply-To: <87zjbxm6fw.fsf@rustcorp.com.au>

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On Wed, Nov 12, 2014 at 02:11:23PM +1030, Rusty Russell wrote:
> Amos Kong <akong@redhat.com> writes:
> > From: Rusty Russell <rusty@rustcorp.com.au>
> >
> > current_rng holds one reference, and we bump it every time we want
> > to do a read from it.
> >
> > This means we only hold the rng_mutex to grab or drop a reference,
> > so accessing /sys/devices/virtual/misc/hw_random/rng_current doesn't
> > block on read of /dev/hwrng.
> >
> > Using a kref is overkill (we're always under the rng_mutex), but
> > a standard pattern.
> >
> > This also solves the problem that the hwrng_fillfn thread was
> > accessing current_rng without a lock, which could change (eg. to NULL)
> > underneath it.
> >
> > v4: decrease last reference for triggering the cleanup
> 
> This doesn't make any sense:
> 
> > +static void drop_current_rng(void)
> > +{
> > +	struct hwrng *rng = current_rng;
> > +
> > +	BUG_ON(!mutex_is_locked(&rng_mutex));
> > +	if (!current_rng)
> > +		return;
> > +
> > +	/* release current_rng reference */
> > +	kref_put(&current_rng->ref, cleanup_rng);
> > +	current_rng = NULL;
> > +
> > +	/* decrease last reference for triggering the cleanup */
> > +	kref_put(&rng->ref, cleanup_rng);
> > +}
> 
> Why would it drop the refcount twice?  This doesn't make sense.
> 
> Hmm, because you added kref_init, which initializes the reference count
> to 1, you created this bug.

I saw some kernel code uses kref_* helper functions, the reference
conter is initialized to 1. Some code didn't use the helper functions
to increase/decrease the reference counter. So I will drop kref_init()
and the second kref_put().

> Leave out the kref_init, and let it naturally be 0 (until, and if, it
> becomes current_rng).  Add a comment if you want.

OK, thanks.

> Thanks,
> Rusty.
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-- 
			Amos.

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