linux-kernel - Re: [RFC PATCH 1/6] kernel: implement queue spinlock API

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Message-ID: <1360281834.28557.73.camel@edumazet-glaptop>
Date:	Thu, 07 Feb 2013 16:03:54 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	paulmck@...ux.vnet.ibm.com, Rik van Riel <riel@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	David Howells <dhowells@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Eric Dumazet <edumazet@...gle.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Manfred Spraul <manfred@...orfullife.com>,
	linux-kernel@...r.kernel.org, john.stultz@...aro.org
Subject: Re: [RFC PATCH 1/6] kernel: implement queue spinlock API

On Thu, 2013-02-07 at 15:58 -0800, Michel Lespinasse wrote:

> No, I think that's doable. The trick would be that once a thread
> acquires the lock, the only remaining use of the node is to receive
> the 'next' pointer if/when another thread starts contending for the
> lock. So if the lock state would need to distinguish between a lock
> that is currently locked but not contended (the next value would then
> be NULL), and a lock that is currently locked and contended (the
> lock->next value is the node that goes after the current lock owner)
> 

It adds yet another memory write to store the node pointer in the
lock...

I suspect it's going to increase false sharing.


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