linux-ext4 - Re: [PATCH] fs/direct-io: avoid data race on ->s_dio_done

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Date:   Wed, 15 Jul 2020 20:19:39 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Dave Chinner <david@...morbit.com>, linux-fsdevel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] fs/direct-io: avoid data race on ->s_dio_done_wq

On Thu, Jul 16, 2020 at 03:47:17AM +0100, Matthew Wilcox wrote:
> On Thu, Jul 16, 2020 at 11:46:56AM +1000, Dave Chinner wrote:
> > And why should we compromise performance on hundreds of millions of
> > modern systems to fix an extremely rare race on an extremely rare
> > platform that maybe only a hundred people world-wide might still
> > use?
> 
> I thought that wasn't the argument here.  It was that some future
> compiler might choose to do something absolutely awful that no current
> compiler does, and that rather than disable the stupid "optimisation",
> we'd be glad that we'd already stuffed the source code up so that it
> lay within some tortuous reading of the C spec.

There are actually many reasons to avoid data races; see
https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_ONCE

> 
> The memory model is just too complicated.  Look at the recent exchange
> between myself & Dan Williams.  I spent literally _hours_ trying to
> figure out what rules to follow.
> 
> https://lore.kernel.org/linux-mm/CAPcyv4jgjoLqsV+aHGJwGXbCSwbTnWLmog5-rxD2i31vZ2rDNQ@mail.gmail.com/
> https://lore.kernel.org/linux-mm/CAPcyv4j2+7XiJ9BXQ4mj_XN0N+rCyxch5QkuZ6UsOBsOO1+2Vg@mail.gmail.com/
> 
> Neither Dan nor I are exactly "new" to Linux kernel development.  As Dave
> is saying here, having to understand the memory model is too high a bar.
> 
> Hell, I don't know if what we ended up with for v4 is actually correct.
> It lokos good to me, but *shrug*
> 
> https://lore.kernel.org/linux-mm/159009507306.847224.8502634072429766747.stgit@dwillia2-desk3.amr.corp.intel.com/

Yes, it's too complicated.  I'm not sure there's much of a solution, though.

Of course, we also have easy-to-use synchronization primitives like mutex,
spinlock, rw_semaphore, etc.  The problems arise when people think they know
better and try to write something more "optimized".  We need to have a higher
bar for accepting changes where the memory model is a concern at all.

- Eric