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Date: Tue, 23 Aug 2016 22:31:03 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <waiman.long@hpe.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Jason Low <jason.low2@hpe.com>,
        Ding Tianhong <dingtianhong@huawei.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <Will.Deacon@arm.com>, Ingo Molnar <mingo@redhat.com>,
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Tim Chen <tim.c.chen@linux.intel.com>, Terry Rudd <terry.rudd@hpe.com>,
        "Paul E. McKenney" <paulmck@us.ibm.com>, Jason Low <jason.low2@hp.com>
Subject: Re: [RFC][PATCH 1/3] locking/mutex: Rework mutex::owner
Message-ID: <20160823203103.GT10153@twins.programming.kicks-ass.net>
References: <20160823124617.015645861@infradead.org>
 <20160823124856.763266868@infradead.org>
 <57BCAF72.7020904@hpe.com>
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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 :-)