linux-kernel - Re: [RFC] Cancellable MCS spinlock rework

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Message-ID: <20140703073148.GT19379@twins.programming.kicks-ass.net>
Date:	Thu, 3 Jul 2014 09:31:48 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Low <jason.low2@...com>
Cc:	torvalds@...ux-foundation.org, paulmck@...ux.vnet.ibm.com,
	mingo@...nel.org, Waiman.Long@...com, davidlohr@...com,
	linux-kernel@...r.kernel.org, riel@...hat.com,
	akpm@...ux-foundation.org, hpa@...or.com, andi@...stfloor.org,
	James.Bottomley@...senpartnership.com, rostedt@...dmis.org,
	tim.c.chen@...ux.intel.com, aswin@...com, scott.norton@...com,
	chegu_vinod@...com
Subject: Re: [RFC] Cancellable MCS spinlock rework

On Wed, Jul 02, 2014 at 09:39:18PM -0700, Jason Low wrote:
> And due to padding, the additional modification below reduces the
> size of struct rw_semaphore by 64 bits on my machine  :)
> 
> 
>  struct rw_semaphore {
>  	long count;
> -	raw_spinlock_t wait_lock;
>  	struct list_head wait_list;
> +	raw_spinlock_t wait_lock;
>  #ifdef CONFIG_SMP
> +	struct optimistic_spin_tail osq; /* spinner MCS lock */
>  	/*
>  	 * Write owner. Used as a speculative check to see
>  	 * if the owner is running on the cpu.
>  	 */
>  	struct task_struct *owner;
> -	struct optimistic_spin_tail osq; /* spinner MCS lock */
>  #endif
>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
>  	struct lockdep_map	dep_map;
> 

Right, that might make sense.

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