From: Eric Biggers <firstname.lastname@example.org> To: Matthew Wilcox <email@example.com> Cc: Dave Chinner <firstname.lastname@example.org>, email@example.com, firstname.lastname@example.org, email@example.com 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://firstname.lastname@example.org/ 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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