linux-kernel - Re: [RFC][PATCH 00/13] percpu rwsem -v2

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Message-ID: <CA+55aFw+pO=qxnust4mwSTDMyZPZrqyJMfwJkNopUh_4xncsLg@mail.gmail.com>
Date:	Mon, 20 Jul 2015 11:44:04 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Daniel Wagner <wagi@...om.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	der.herr@...r.at, Davidlohr Bueso <dave@...olabs.net>,
	Rik van Riel <riel@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Jeff Layton <jlayton@...chiereds.net>
Subject: Re: [RFC][PATCH 00/13] percpu rwsem -v2

On Sun, Jul 19, 2015 at 10:53 PM, Daniel Wagner <wagi@...om.org> wrote:
>
> Turning them back into arch_spinlock_t gives almost the same numbers as
> with spinlock_t.
>
> Then Peter suggested to change the code to
>
>         preempt_disable();
>         spin_unlock();
>         preempt_enable_no_resched();
>
> to verify if arch_spin_lock() is buggy and does not disable preemption
> and we see a lock holder preemption on non virt setups.

Hmm. "arch_spin_lock()" isn't _supposed_ to disable preemption. The
caller should do that (possibly by disabling interrupts). See
include/linux/spinlock_api_smp.h for details.

But yes, that's a *very* subtle difference between "arch_spin_lock()"
and "spin_lock()". The former doesn't do lockdep or other debugging
and it doesn't disable preemption. So they are not interchangeable.

The current lglocks uses arch_spin_lock exactly because it does not
*want* lockdep tracking (it does its own) and because it does its own
preemption handling.

So saying "verify if arch_spin_lock() is buggy and does not disable
preemption" is complete BS. If arch_spin_lock() were to disable
preemption, _that_ would be a bug.

                Linus
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