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Date: Fri, 22 Oct 2010 13:48:34 +1100
From: Nick Piggin <npiggin@kernel.dk>
To: Nick Piggin <npiggin@kernel.dk>
Cc: Al Viro <viro@ZenIV.linux.org.uk>, Dave Chinner <david@fromorbit.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: Inode Lock Scalability V7 (was V6)
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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();
  }
}