linux-kernel - Re: [PATCH 2/5] new fscache interface to check cache consistency

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Date:	Wed, 04 Sep 2013 17:24:03 +0100
From:	David Howells <dhowells@...hat.com>
To:	Hongyi Jia <jiayisuse@...il.com>, milosz@...in.com
Cc:	dhowells@...hat.com, ceph-devel@...r.kernel.org, sage@...tank.com,
	zheng.z.yan@...el.com, linux-cachefs@...hat.com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] new fscache interface to check cache consistency

Hongyi Jia <jiayisuse@...il.com> wrote:

> +bool __fscache_check_consistency(struct fscache_cookie *cookie)
> +{
> +	struct fscache_object *object;
> +
> +	if (cookie->def->type != FSCACHE_COOKIE_TYPE_DATAFILE)
> +		return false;
> +
> +	if (hlist_empty(&cookie->backing_objects))
> +		return false;
> +
> +	object = hlist_entry(cookie->backing_objects.first,
> +			struct fscache_object, cookie_link);
> +
> +	return object->cache->ops->check_consistency(object);
> +}

Hmmm...  This isn't actually safe.  You have to either:

 (1) hold cookie->lock whilst touching the object pointer when coming from the
     netfs side, or:

 (2) set up an operation to do this (as, say, __fscache_alloc_page() does).

The problem is that you have nothing to defend against the object being
withdrawn by the cache under you.

David
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