linux-kernel - Re: qrwlock && read-after-read

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Message-ID: <CA+55aFx-mQeayVa-Cq=GYNgPQRrct-B7HrrazhdCB7KebgD=aw@mail.gmail.com>
Date:	Tue, 4 Aug 2015 06:51:43 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Oleg Nesterov <oleg@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	Ingo Molnar <mingo@...hat.com>,
	"Paul E.McKenney" <paulmck@...ux.vnet.ibm.com>,
	Waiman Long <Waiman.Long@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: qrwlock && read-after-read

On Tue, Aug 4, 2015 at 6:10 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>
> These (new) semantics were very much on purpose and suggested by Linus
> IIRC.

Well, I'll not take all the blame.

I refused to have something that broke the tasklist lock, so the "irq
users nest" was a requirement.

I also refused to have the original version that made this a per-lock
explicit and confusing choice, which in turn required changes to
existing users, and that made the interface more complex in ways that
didn't actually help anybody.

So it's not like I love the current semantics, but at least they are
realistic and can work. I agree that teaching lockdep to check for
this would be a good idea, because the semantics _are_ subtle.

(And I'm not 100% convinced we needed the fair model at all, but
fairness does end up being a good thing _if_ it works).

                   Linus
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