linux-kernel - Re: [PATCH 3/3] rtmutex: Reduce top-waiter blocking on a lock

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Date:	Mon, 14 Mar 2016 14:23:56 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	tglx@...utronix.de, mingo@...nel.org, bigeasy@...utronix.de,
	umgwanakikbuti@...il.com, paulmck@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 3/3] rtmutex: Reduce top-waiter blocking on a lock

On Tue, Mar 08, 2016 at 10:20:23AM -0800, Davidlohr Bueso wrote:
>  	lock->owner = (void *) RT_MUTEX_HAS_WAITERS;

>  static inline struct task_struct *rt_mutex_owner(struct rt_mutex *lock)
>  {
>  	return (struct task_struct *)
> -		((unsigned long)lock->owner & ~RT_MUTEX_OWNER_MASKALL);
> +		((unsigned long)READ_ONCE(lock->owner) & ~RT_MUTEX_OWNER_MASKALL);
>  }

If you READ_ONCE(), you should also WRITE_ONCE(), because while the
write is under the proper locks, our friendly compiler might still
choose to emit the store in a random sequence of byte stores, rendering
our READ_ONCE() pointless.