linux-kernel - Re: [PATCH 02/11] writeback: switch to per-bdi threads for flushing data

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Message-ID: <20090520121629.GW11363@kernel.dk>
Date:	Wed, 20 May 2009 14:16:30 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	chris.mason@...cle.com, david@...morbit.com, hch@...radead.org,
	akpm@...ux-foundation.org, yanmin_zhang@...ux.intel.com
Subject: Re: [PATCH 02/11] writeback: switch to per-bdi threads for
	flushing data

On Wed, May 20 2009, Jan Kara wrote:
> On Wed 20-05-09 13:32:34, Jens Axboe wrote:
> > On Wed, May 20 2009, Jan Kara wrote:
> > >   Hi Jens,
> > > 
> > >   a few comments here. Mainly, I still don't think the sys_sync() is
> > > working right - see comments below.
> > 
> > Thanks! I took the liberty of killing some of the code in between, to
> > make it easier to see.
> > 
> > > > +void bdi_writeback_all(struct super_block *sb, long nr_pages)
> > > > +{
> > > > +	struct backing_dev_info *bdi;
> > > > +
> > > > +	rcu_read_lock();
> > > > +
> > > > +restart:
> > > > +	list_for_each_entry_rcu(bdi, &bdi_list, bdi_list) {
> > >   Isn't the RCU list here a bit overengineering? AFAICS we use the list
> > > only here and if I'm grepping right, generic_sync_sb_inodes() is currently
> > > only used for data integrity sync (after your patches) from fs-writeback.c
> > > and by UBIFS to do equivalent of writeback_inodes(). So simple spinlock
> > > guarding the list should be just fine. Or am I missing something?
> > 
> > Sure, we could. But it's really not that much of a difference,
> > implementation wise.
>   Yeah. It's just that when I see RCU, I'm a bit cautious what's going on.
> When I see spinlock, everything is simple and clear ;). And I'm in favor of
> using the simplest synchronization primitive that does it's work good
> enough ;).

It's a fine rule, I agree ;-)

I'll take another look at this when splitting the sync paths.

-- 
Jens Axboe

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