linux-kernel - Re: [PATCH RFC v2 1/2] qspinlock: Introducing a 4-byte queue spinlock implementation

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Date:	Thu, 29 Aug 2013 11:24:57 -0400
From:	Waiman Long <waiman.long@...com>
To:	Alexander Fyodorov <halcy@...dex.ru>
CC:	linux-kernel <linux-kernel@...r.kernel.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH RFC v2 1/2] qspinlock: Introducing a 4-byte queue spinlock
 implementation

On 08/27/2013 08:09 AM, Alexander Fyodorov wrote:
>> I also thought that the x86 spinlock unlock path was an atomic add. It
>> just comes to my realization recently that this is not the case. The
>> UNLOCK_LOCK_PREFIX will be mapped to "" except for some old 32-bit x86
>> processors.
> Hmm, I didn't know that. Looking through Google found these rules for x86 memory ordering:
>    * Loads are not reordered with other loads.
>    * Stores are not reordered with other stores.
>    * Stores are not reordered with older loads.
> So x86 memory model is rather strict and memory barrier is really not needed in the unlock path - xadd is a store and thus behaves like a memory barrier, and since only lock's owner modifies "ticket.head" the "add" instruction need not be atomic.
>
> But this is true only for x86, other architectures have more relaxed memory ordering. Maybe we should allow arch code to redefine queue_spin_unlock()? And define a version without smp_mb() for x86?

What I have been thinking is to set a flag in an architecture specific 
header file to tell if the architecture need a memory barrier. The 
generic code will then either do a smp_mb() or barrier() depending on 
the presence or absence of the flag. I would prefer to do more in the 
generic code, if possible.

Regards,
Longman
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