linux-kernel - Re: Inode Lock Scalability V7 (was V6)

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Date:	Fri, 22 Oct 2010 15:48:12 +1100
From:	Nick Piggin <npiggin@...nel.dk>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Nick Piggin <npiggin@...nel.dk>,
	Dave Chinner <david@...morbit.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Inode Lock Scalability V7 (was V6)

On Fri, Oct 22, 2010 at 04:12:11AM +0100, Al Viro wrote:
> 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.

That sucks, it requires more acquiring and dropping of i_lock and
it hits single threaded performance. I looked at that.

But it also loses the i_lock = inode_lock property.

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