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

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Message-ID: <20160824095659.GC16944@arm.com>
Date:   Wed, 24 Aug 2016 10:56:59 +0100
From:   Will Deacon <will.deacon@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Waiman Long <waiman.long@....com>,
        Jason Low <jason.low2@....com>,
        Ding Tianhong <dingtianhong@...wei.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        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 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@...radead.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