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

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@...e.com>
SUSE Labs, CR

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