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

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Date:   Tue, 23 Aug 2016 22:31:03 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <waiman.long@....com>
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 Tue, Aug 23, 2016 at 04:17:54PM -0400, Waiman Long wrote:
> 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.

Right you are.. didn't get around to looking at layout yet. Just barely
got it to compile and boot :-)