Re: Inode Lock Scalability V7 (was V6)

Al Viro
2010-10-22T03:12:14Z

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.