From: Eric Biggers <ebiggers@kernel.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: Dave Chinner <david@fromorbit.com>,
linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org,
linux-ext4@vger.kernel.org
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: <20200716024717.GJ12769@casper.infradead.org>
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
next prev parent 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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