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From: Eric Biggers <>
To: Matthew Wilcox <>
Cc: Dave Chinner <>,,,
Subject: Re: [PATCH] fs/direct-io: avoid data race on ->s_dio_done_wq
Date: Wed, 15 Jul 2020 20:19:39 -0700	[thread overview]
Message-ID: <20200716031939.GF1167@sol.localdomain> (raw)
In-Reply-To: <>

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

> 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.
> 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*

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

  reply	other threads:[~2020-07-16  3:19 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-13  3:33 Eric Biggers
2020-07-15  1:30 ` Dave Chinner
2020-07-15  2:37   ` Eric Biggers
2020-07-15  8:01     ` Dave Chinner
2020-07-15 16:13       ` Eric Biggers
2020-07-15 16:41         ` Darrick J. Wong
2020-07-16  1:46         ` Dave Chinner
2020-07-16  2:39           ` Eric Biggers
2020-07-16  2:47           ` Matthew Wilcox
2020-07-16  3:19             ` Eric Biggers [this message]
2020-07-16  5:33             ` Eric Biggers
2020-07-16  8:16               ` Dave Chinner
2020-07-17  1:36                 ` Darrick J. Wong

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