linux-ext4 - Re: [PATCH 4/4] ext4: Wait for proper transaction commit on fsync

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Message-ID: <20091104155344.GC21088@duck.suse.cz>
Date:	Wed, 4 Nov 2009 16:53:44 +0100
From:	Jan Kara <jack@...e.cz>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org, tytso@....edu
Subject: Re: [PATCH 4/4] ext4: Wait for proper transaction commit on fsync

On Wed 04-11-09 15:57:22, Andi Kleen wrote:
> Jan Kara <jack@...e.cz> writes:
> >  	ext4_io_end_t *cur_aio_dio;
> > +
> > +	/*
> > +	 * Transactions that contain inode's metadata needed to complete
> > +	 * fsync and fdatasync, respectively.
> > +	 */
> > +	atomic_t i_sync_tid;
> > +	atomic_t i_datasync_tid;
> 
> This might be a stupid question, but the atomic implies you don't hold
> any kind of reference to the transaction. So what prevents these IDs
> from wrapping while in there? Given it would be probably take a long
> time today, but might be not fully future proof to very fast future IO
> devices.
  Yes, IDs can wrap. But journalling layer actually compares them so that
wrapping does not matter unless you manage to do 2^31 transactions
inbetween. So we are still a few orders of magnitude safe with current hw.

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