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

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Date:	Fri, 22 Oct 2010 13:41:52 +1100
From:	Nick Piggin <npiggin@...nel.dk>
To:	Nick Piggin <npiggin@...nel.dk>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	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 01:34:44PM +1100, Nick Piggin wrote:
> On Fri, Oct 22, 2010 at 03:20:10AM +0100, Al Viro wrote:
> > On Fri, Oct 22, 2010 at 11:45:40AM +1100, Nick Piggin wrote:
> > majority already checks for I_FREEING/I_WILL_FREE, refusing to pick such
> > inodes.  It's not an accidental subtle property of the code, it's bloody
> > fundamental.
> 
> I didn't miss that, and I agree that at the point of my initial lock
> break up, the locking is "wrong". Whether you correct it by changing
> the lock ordering or by using RCU to do lookups is something I want to
> debate further.
> 
> I think it is natural to be able to lock the inode and have it lock the
> icache state.

Importantly, to be able to manipulate the icache state in any number of
steps, under a consistent lock. Exactly like we have with inode_lock
today.

Stepping away from that, adding code to handle new concurrencies, before
inode_lock is able to be lifted is just wrong.

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).
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