linux-kernel - Re: [PATCH 06/17] fs: icache lock lru/writeback lists

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Date:	Wed, 6 Oct 2010 09:30:22 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/17] fs: icache lock lru/writeback lists

On Fri, Oct 01, 2010 at 02:01:03AM -0400, Christoph Hellwig wrote:
> On Wed, Sep 29, 2010 at 10:18:38PM +1000, Dave Chinner wrote:
> > From: Nick Piggin <npiggin@...e.de>
> > 
> > The inode moves between different lists protected by the inode_lock. Introduce
> > a new lock that protects all of the lists (dirty, unused, in use, etc) that the
> > inode will move around as it changes state. As this is mostly a list for
> > protecting the writeback lists, name it wb_inode_list_lock and nest all the
> > list manipulations in this lock inside the current inode_lock scope.
> 
> As a band-aid to get rid of the inode_lock this might be fine, but I
> don't really like it.  For one all the list are per-bdi_writeback, so
> the lock should be as well.  Second the lock is held over far too long
> periods during writeback, which leads to a lot of whacky trylock
> operations and unlock and sleep cycles inside it.  In practice we only
> need it in the places where we manipulate the lists.

per-bdi writeback lock won't work with the patch set as it stands -
it also protects the LRU which is a global list. I'll have to pull
back another patch to split the LRU and IO lists to make this lock
per-bdi.

> Also it feels like it really should nest outside i_lock, not inside it,
> but I need to look more deeply to figure why that might not easily be
> possible.

Yeah, I'm trying to rework the patch series to not nest anything
inside i_lock. The more I look at all that trylock stuff, the more
my eyes bleed....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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