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

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Date:	Thu, 20 Feb 2014 12:54:07 -0500
From:	Waiman Long <waiman.long@...com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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 02/19/2014 03:17 PM, Linus Torvalds wrote:
> On Wed, Feb 19, 2014 at 11:24 AM, Waiman Long<waiman.long@...com>  wrote:
>> We do have a CONFIG_PARAVIRT macro that turns on or off PV support. The
>> queue spinlock can be easily changed into an unfair lock which allows lock
>> stealing. We could have a config option to make it unfair in the PARAVIRT
>> environment, but I don't think Linus like the idea of an unfair lock.
> I could care less for the paravirt case. As long as the native case
> ends up being sane (even when CONFIG_PARAVIRT is set at compile time),
> I'm fine.
>
> When actually running in a paravirtualized environment, locks are
> always going to have problems.
>
>                    Linus

I think we could implement 2 versions of _raw_spin_lock. The primary one 
is fair and the other one is unfair with a jump label that jump from the 
fair version to the unfair version. At boot time, the kernel can check 
to see it is really running in a PV environment and then activate the 
jump label to use the unfair version.

Now the key is how to detect if a kernel is really running in a PV 
environment. I need to ask some virtualization experts on that.

-Longman
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