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From: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: Inode Lock Scalability V7 (was V6)
Date: Fri, 22 Oct 2010 04:12:11 +0100
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Cc: Dave Chinner <david@fromorbit.com>, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org
To: Nick Piggin <npiggin@kernel.dk>
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On Fri, Oct 22, 2010 at 01:48:34PM +1100, Nick Piggin wrote:
> On Fri, Oct 22, 2010 at 01:41:52PM +1100, Nick Piggin wrote:
> > The locking in my lock break patch is ugly and wrong, yes. But it is
> > always an intermediate step. I want to argue that with RCU inode work
> > *anyway*, there is not much point to reducing the strength of the
> > i_lock property because locking can be cleaned up nicely and still
> > keep i_lock ~= inode_lock (for a single inode).
> 
> The other thing is that with RCU, the idea of locking an object in
> the data structure with a per object lock actually *is* much more
> natural. It's hard to do it properly with just a big data structure
> lock.
> 
> If I want to take a reference to an inode from a data structre, how
> to do it with RCU?
> 
> rcu_read_lock()
> list_for_each(inode) {
>   spin_lock(&big_lock); /* oops, might as well not even use RCU then */
>   if (!unhashed) {
>     iget();
>   }
> }

Huh?  Why the hell does it have to be a big lock?  You grab ->i_lock,
then look at the damn thing.  You also grab it on eviction from the
list - *inside* the lock used for serializing the write access to
your RCU list.