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Date: Wed, 24 Aug 2016 10:56:59 +0100
From: Will Deacon <will.deacon@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Waiman Long <waiman.long@hpe.com>, Jason Low <jason.low2@hpe.com>,
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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        "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: <20160824095659.GC16944@arm.com>
References: <20160823124617.015645861@infradead.org>
 <20160823124856.763266868@infradead.org>
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On Tue, Aug 23, 2016 at 02:46:18PM +0200, Peter Zijlstra wrote:
> There's a number of iffy in mutex because mutex::count and
> mutex::owner are two different fields; this too is the reason
> MUTEX_SPIN_ON_OWNER and DEBUG_MUTEX are mutually exclusive.
> 
> Cure this by folding them into a single atomic_long_t field.
> 
> This nessecairly kills all the architecture specific mutex code.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

[...]

>  void __sched mutex_unlock(struct mutex *lock)
>  {
> -	/*
> -	 * The unlocking fastpath is the 0->1 transition from 'locked'
> -	 * into 'unlocked' state:
> -	 */
> -#ifndef CONFIG_DEBUG_MUTEXES
> -	/*
> -	 * When debugging is enabled we must not clear the owner before time,
> -	 * the slow path will always be taken, and that clears the owner field
> -	 * after verifying that it was indeed current.
> -	 */
> -	mutex_clear_owner(lock);
> +	unsigned long owner;
> +
> +#ifdef CONFIG_DEBUG_MUTEXES
> +	DEBUG_LOCKS_WARN_ON(__mutex_owner(lock) != current);
>  #endif
> -	__mutex_fastpath_unlock(&lock->count, __mutex_unlock_slowpath);
> -}
>  
> +	owner = atomic_long_read(&lock->owner);
> +	for (;;) {
> +		unsigned long old;
> +
> +		old = atomic_long_cmpxchg_release(&lock->owner, owner, owner & 0x03);
> +		if (old == owner)
> +			break;
> +
> +		owner = old;
> +	}

Can you rewrite this using atomic_long_fetch_and_release?

Will