linux-kernel - Re: [PATCHv5 04/16] VFS: add memory barrier to sb_mark_clean and sb_mark

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Date:	Wed, 09 Jun 2010 19:36:50 +0300
From:	Artem Bityutskiy <Artem.Bityutskiy@...ia.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCHv5 04/16] VFS: add memory barrier to sb_mark_clean and
 sb_mark_dirty

On Sun, 2010-06-06 at 17:50 +0300, Artem Bityutskiy wrote:
> From: Artem Bityutskiy <Artem.Bityutskiy@...ia.com>
> 
> The proper way for file-systems to synchronize the superblock
> should be as follows:
> 
> 1. when modifying the SB, first modify it, then mark it as dirty;
> 2. when synchronizing the SB, first mark as clean, then start
>    synchronizing.
> 
> And to make ensure the order, we need memory barriers in 'sb_mark_clean()'
> and 'sb_mark_dirty()'.

I believe this stuff is a separate story, and should be handled
separately. I'll keep this separately from the 'sync_supers()' wakes up
optimization.

I actually now cannot prove myself whether these smp_mb()'s I added in
this patch make sense or not, and whether the races in FSes I was trying
to address can be addressed without spinlocks. Really dunno - but I will
keep trying to get better understanding. Reading
Documentation/memory-barriers.txt and some McKenny's docs only did not
help so far :-)

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

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