linux-kernel - Re: [RFC][PATCH 1/3] locking/mutex: Rework mutex::owner

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Message-ID: <57BCAF72.7020904@hpe.com>
Date:   Tue, 23 Aug 2016 16:17:54 -0400
From:   Waiman Long <waiman.long@....com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Jason Low <jason.low2@....com>,
        Ding Tianhong <dingtianhong@...wei.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        Will Deacon <Will.Deacon@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Imre Deak <imre.deak@...el.com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        "Tim Chen" <tim.c.chen@...ux.intel.com>,
        Terry Rudd <terry.rudd@....com>,
        "Paul E. McKenney" <paulmck@...ibm.com>,
        Jason Low <jason.low2@...com>
Subject: Re: [RFC][PATCH 1/3] locking/mutex: Rework mutex::owner

On 08/23/2016 08:46 AM, Peter Zijlstra wrote:
>   /*
>    * Simple, straightforward mutexes with strict semantics:
> @@ -48,13 +49,9 @@
>    *   locks and tasks (and only those tasks)
>    */
>   struct mutex {
> -	/* 1: unlocked, 0: locked, negative: locked, possible waiters */
> -	atomic_t		count;
> +	atomic_long_t		owner;
>   	spinlock_t		wait_lock;
>   	struct list_head	wait_list;
> -#if defined(CONFIG_DEBUG_MUTEXES) || defined(CONFIG_MUTEX_SPIN_ON_OWNER)
> -	struct task_struct	*owner;
> -#endif
>   #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
>   	struct optimistic_spin_queue osq; /* Spinner MCS lock */
>   #endif

I think you should put the wait_lock and osq next to each other to save 
8 bytes in space on 64-bit machines.

Cheers,
Longman