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 [off-list ref]
SUSE Labs, CR
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