linux-ext4 - Re: per inode fsync optimization question

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Message-ID: <87zjxfps5u.fsf@openvz.org>
Date:	Wed, 03 Apr 2013 19:09:33 +0400
From:	Dmitry Monakhov <dmonakhov@...nvz.org>
To:	Jan Kara <jack@...e.cz>
Cc:	ext4 development <linux-ext4@...r.kernel.org>,
	Jan Kara <jack@...e.cz>
Subject: Re: per inode fsync optimization question

On Wed, 3 Apr 2013 16:50:55 +0200, Jan Kara <jack@...e.cz> wrote:
> On Wed 03-04-13 18:21:46, Dmitry Monakhov wrote:
> > inode store i_sync_tid and i_datasync_tid  in order to optimize journal
> > flushes and wait for commits only when necessary, but
> > fields are declared as tid_t(not atomic_t as it done in ext3) so we
> > have not synchronization between readers and writers, so gcc and cpu
> > is allowed to perform prefetch, cache and other stuff.
> > Looks like a bug, right?
>   Reads and writes to atomic_t aren't guaranteed to be any kind of a
> barrier (if fact they are compiled as simple stores and loads on x86). Only
> arithmetic operations on atomic types are special. So using tid_t is just
> fine.
Ok but what about prefetching?
Compiler is allowed to prefetch on early stage ?
should we use ACCESS_ONCE() or wmb() and rmb() here?


> 
> 								Honza
> -- 
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR
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