linux-kernel - Re: [PATCH v10 09/19] qspinlock: Prepare for unfair lock support

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Message-ID: <20140508190649.GR2844@laptop.programming.kicks-ass.net>
Date:	Thu, 8 May 2014 21:06:49 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@...com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-arch@...r.kernel.org,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
	Paolo Bonzini <paolo.bonzini@...il.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	Scott J Norton <scott.norton@...com>,
	Chegu Vinod <chegu_vinod@...com>
Subject: Re: [PATCH v10 09/19] qspinlock: Prepare for unfair lock support

On Wed, May 07, 2014 at 11:01:37AM -0400, Waiman Long wrote:
> If unfair lock is supported, the lock acquisition loop at the end of
> the queue_spin_lock_slowpath() function may need to detect the fact
> the lock can be stolen. Code are added for the stolen lock detection.
> 
> A new qhead macro is also defined as a shorthand for mcs.locked.

NAK, unfair should be a pure test-and-set lock.

>  /**
>   * get_qlock - Set the lock bit and own the lock
> - * @lock: Pointer to queue spinlock structure
> + * @lock : Pointer to queue spinlock structure
> + * Return: 1 if lock acquired, 0 otherwise
>   *
>   * This routine should only be called when the caller is the only one
>   * entitled to acquire the lock.
>   */
> -static __always_inline void get_qlock(struct qspinlock *lock)
> +static __always_inline int get_qlock(struct qspinlock *lock)
>  {
>  	struct __qspinlock *l = (void *)lock;
>  
>  	barrier();
>  	ACCESS_ONCE(l->locked) = _Q_LOCKED_VAL;
>  	barrier();
> +	return 1;
>  }

and here you make a horribly named function more horrible;
try_set_locked() is that its now.
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