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Date: Mon, 6 Aug 2018 22:20:17 +0200
From: Jan Kara <jack@suse.cz>
Subject: Re: SLAB_TYPESAFE_BY_RCU without constructors (was Re: [PATCH v4
 13/17] khwasan: add hooks implementation)
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To: Dmitry Vyukov <dvyukov@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, Christoph Lameter <cl@linux.com>, Andrey Ryabinin <aryabinin@virtuozzo.com>, Theodore Ts'o <tytso@mit.edu>, Jan Kara <jack@suse.com>, linux-ext4@vger.kernel.org, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Pablo Neira Ayuso <pablo@netfilter.org>, Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>, Florian Westphal <fw@strlen.de>, David Miller <davem@davemloft.net>, NetFilter <netfilter-devel@vger.kernel.org>, coreteam@netfilter.org, Network Development <netdev@vger.kernel.org>, Gerrit Renker <gerrit@erg.abdn.ac.uk>, dccp@vger.kernel.org, Jani Nikula <jani.nikula@linux.intel.com>, Joonas Lahtinen <joonas.lahtinen@linux.intel.com>, Rodrigo Vivi <rodrigo.vivi@intel.com>, Dave Airlie <airlied@linux.ie>, intel-gfx <intel-gfx@lists.freedesktop.org>, DRI <dri-devel@lists.freedesktop.org>, Eric Dumazet <edumazet@google.com>, Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>, Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>, Ursula Braun <ubraun@linux.ibm.com>, linux-s390 <linux-s390@vger.kernel.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Andrew Morton <akpm@linux-foundation.org>, linux-mm <linux-mm@kvack.org>, Andrey Konovalov <andreyknvl@google.com>

On Wed 01-08-18 10:46:35, Dmitry Vyukov wrote:
> I guess it would be useful to have such extensive comment for each
> SLAB_TYPESAFE_BY_RCU use explaining why it is needed and how all the
> tricky aspects are handled.
> 
> For example, the one in jbd2 is interesting because it memsets the
> whole object before freeing it into SLAB_TYPESAFE_BY_RCU slab:
> 
> memset(jh, JBD2_POISON_FREE, sizeof(*jh));
> kmem_cache_free(jbd2_journal_head_cache, jh);
> 
> I guess there are also tricky ways how it can all work in the end
> (type-stable state is only a byte, or we check for all possible
> combinations of being overwritten with JBD2_POISON_FREE). But at first
> sight it does look fishy.

The RCU access is used from a single place:

fs/jbd2/transaction.c: jbd2_write_access_granted()

There are also quite some comments explaining why what it does is safe. The
overwrite by JBD2_POISON_FREE is much older than this RCU stuff (honestly I
didn't know about it until this moment) and has nothing to do with the
safety of RCU access.

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR