Re: [PATCH v2] vfs: Don't exchange "short" filenames unconditionally.

[Al Viro <viro@ZenIV.linux.org.uk>]

On Sat, Sep 27, 2014 at 07:31:39PM +0100, Al Viro wrote:

> We can get the long name cases right, and I agree that it'll make the
> things nicer, but it might take a couple of days to get right.  The thing
> I'm concerned about is not screwing DCACHE_RCUACCESS up.

FWIW, I suspect that the right approach is to put refcount + rcu_head in
front of external name and do the following:
	* __d_free() checks if we have an external name, gets its containing
structure and does if (atomic_dec_and_test(&name->count)) kfree(name);
	* switch_names() in non-exchange case (I'd probably call it copy_name,
not move_names, but anyway) sets DCACHE_RCUACCESS on target (source has
already gotten it from __d_rehash()), increments refcount on target's name
if external and, if the source old name is external, decrements its refcount
and calls kfree_rcu() if it has hit zero.

AFAICS, it guarantees that we'll schedule an RCU callback on name's rch_head
at most once, that we won't free it while RCU callback on it is scheduled
and we won't free it until a grace period has expired since the last time
it had been referenced by observable dentries.  Do you see any holes in that?