linux-kernel - Re: POSIX violation by writeback error

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Message-ID: <20180927234858.507ea60b@alans-desktop>
Date:   Thu, 27 Sep 2018 23:48:58 +0100
From:   Alan Cox <gnomes@...rguk.ukuu.org.uk>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Jeff Layton <jlayton@...hat.com>,
        焦晓冬 <milestonejxd@...il.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Rogier Wolff <R.E.Wolff@...Wizard.nl>
Subject: Re: POSIX violation by writeback error

On Wed, 26 Sep 2018 17:49:09 -0400
"Theodore Y. Ts'o" <tytso@....edu> wrote:

> On Wed, Sep 26, 2018 at 07:10:55PM +0100, Alan Cox wrote:
> > In almost all cases you don't care so you wouldn't use it. In those cases
> > where it might matter it's almost always the case that a reader won't
> > consume it before it hits the media.
> > 
> > That's why I suggested having an fbarrier() so you can explicitly say 'in
> > the even that case does happen then stall and write it'. It's kind of
> > lazy fsync. That can be used with almost no cost by things like mail
> > daemons.  
> 
> How could mail daemons use it?  They *have* to do an fsync() before
> they send a 2xx SMTP return code.

Point - so actually it would be less useful

> > Another way given that this only really makes sense with locks
> > is to add that fbarrier notion as a file locking optional semantic so you
> > can 'unlock with barrier' and 'lock with barrier honoured'  
> 
> I'm not sure what you're suggesting?

If someone has an actual case you could in theory constrain it to a range
specified in a file lock and only between two people who care. That said
seems like a lot of complexity to make a case nobody cares about only
affect people who care about it

Alan