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Date:   Mon, 6 Aug 2018 22:20:17 +0200
From:   Jan Kara <jack@suse.cz>
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Christoph Lameter <cl@linux.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
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Subject: Re: SLAB_TYPESAFE_BY_RCU without constructors (was Re: [PATCH v4
 13/17] khwasan: add hooks implementation)
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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