linux-kernel - Re: [PATCH v4 0/3] qspinlock: Introducing a 4-byte queue spinlock

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Message-ID: <CA+55aFyJTOwnSJxc_qeHxKGoZ71xo3UWc4DrAJ5=OAtOvG7_CA@mail.gmail.com>
Date:	Thu, 20 Feb 2014 10:42:38 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Waiman Long <waiman.long@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michel Lespinasse <walken@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Rik van Riel <riel@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	George Spelvin <linux@...izon.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Daniel J Blueman <daniel@...ascale.com>,
	Alexander Fyodorov <halcy@...dex.ru>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Scott J Norton <scott.norton@...com>,
	Thavatchai Makphaibulchoke <thavatchai.makpahibulchoke@...com>
Subject: Re: [PATCH v4 0/3] qspinlock: Introducing a 4-byte queue spinlock

On Thu, Feb 20, 2014 at 9:54 AM, Waiman Long <waiman.long@...com> wrote:
>
> I think we could implement 2 versions of _raw_spin_lock.

Yup. Or rather,  I'd suggest implement just one version of
arch_spin_lock(), but at the top of it you do something like

   #if CONFIG_PARAVIRT_SPINLOCK
        if (static_key_false(&unfair_spinlocks)) {
           .. do paravirt unfair lock version ..
        }
   #endif

which should basically generate almost-perfect code:  it's one extra
no-op for the native case if CONFIG_PARAVIRT_SPINLOCK is on, which
turns into a branch for the unfair version for paravirtualization.

Or something like that.

              Linus
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