Re: [PATCH v4 1/1] dcache: Translating dentry into pathname withouttaking rename_lock

[Linus Torvalds]

On Mon, Sep 9, 2013 at 10:29 AM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>
> I'm not sure I like mixing rcu_read_lock() into that - d_path() and friends
> can do that themselves just fine (it needs to be taken when seq is even),
> and e.g. d_walk() doesn't need it at all.  Other than that, I'm OK with
> this variant.

Hmm.. I think you need the RCU read lock even when you get the write_seqlock().

Yes, getting the seqlock for write implies that you get a spinlock and
in many normal circumstances that basically is equvalent to being
rcu-locked, but afaik in some configurations that is *not* sufficient
protection against an RCU grace period on another CPU. You need to do
a real rcu_read_lock that increments that whole rcu_read_lock_nesting
level, which a spinlock won't do.

And while the rename sequence lock protects against _renames_, it does
not protect against just plain dentries getting free'd under memory
pressure.

So I think the RCU-readlockness really needs to be independent of the
sequence lock.

            Linus
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