linux-ext4 - Re: RFC: hold i_rwsem until aio completes

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Date:   Tue, 4 Feb 2020 10:02:09 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Christoph Hellwig <hch@....de>
Cc:     linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-ext4@...r.kernel.org, cluster-devel@...hat.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: RFC: hold i_rwsem until aio completes

On Mon, Feb 03, 2020 at 06:46:41PM +0100, Christoph Hellwig wrote:
> On Sat, Jan 18, 2020 at 08:28:38PM +1100, Dave Chinner wrote:
> > I think it's pretty gross, actually. It  makes the same mistake made
> > with locking in the old direct IO code - it encodes specific lock
> > operations via flags into random locations in the DIO path. This is
> > a very slippery slope, and IMO it is an layering violation to encode
> > specific filesystem locking smeantics into a layer that is supposed
> > to be generic and completely filesystem agnostic. i.e.  this
> > mechanism breaks if a filesystem moves to a different type of lock
> > (e.g. range locks), and history teaches us that we'll end up making
> > a horrible, unmaintainable mess to support different locking
> > mechanisms and contexts.
> > 
> > I think that we should be moving to a model where the filesystem
> > provides an unlock method in the iomap operations structure, and if
> > the method is present in iomap_dio_complete() it gets called for the
> > filesystem to unlock the inode at the appropriate point. This also
> > allows the filesystem to provide a different method for read or
> > write unlock, depending on what type of lock it held at submission.
> > This gets rid of the need for the iomap code to know what type of
> > lock the caller holds, too.
> 
> I'd rather avoid yet another method.  But I think with a little
> tweaking we can move the unlock into the ->end_io method.

That would work, too :)

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com