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

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Message-ID: <87zjbxm6fw.fsf@rustcorp.com.au>
Date:	Wed, 12 Nov 2014 14:11:23 +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
Subject: Re: [PATCH v4 3/6] hw_random: use reference counts on each struct hwrng.

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.

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

Thanks,
Rusty.
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