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

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Date:	Mon, 17 Nov 2014 23:20:53 +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
Subject: Re: [PATCH v4 3/6] hw_random: use reference counts on each struct
 hwrng.

On Wed, Nov 12, 2014 at 02:11:23PM +1030, Rusty Russell wrote:
> Amos Kong <akong@...hat.com> writes:
> > From: Rusty Russell <rusty@...tcorp.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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