Re: Lockless page cache test results

[Linus Torvalds <torvalds@osdl.org>]

On Fri, 28 Apr 2006, Nick Piggin wrote:
> > 
> > See __d_lookup() for details.
> 
> Yes I see. Perhaps a seqlock could do the trick (hmm, there already is one),
> however we still have to increment the refcount, so there'll always be a
> shared cacheline.

Actually, the thing I'd really _like_ to see is not even incrementing the 
refcount for intermediate directories (and those are actually the most 
common case).

It should be possible in theory to do a lookup of a long path all using 
the rcu_read_lock, and only do the refcount increment (and then you might 
as well do the d_lock thing) for the final component of the path.

Of course, it's not possible right now. We do each component separately, 
and we very much depend on the d_lock. For some things, we _have_ to do it 
that way (revalidation etc), so the "possible in theory" isn't always even 
true.

And every time I look at it, I decide that it's too damn complex, and the 
end result would look horrible, and that I'd probably get it wrong anyway.

Still, I've _looked_ at it several times.

		Linus

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