linux-kernel - Re: [PATCH v11 0/4] Introducing a queue read/write lock implementation

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Date:	Thu, 30 Jan 2014 16:53:44 +0100
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>, Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michel Lespinasse <walken@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Rik van Riel <riel@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	George Spelvin <linux@...izon.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>, aswin@...com,
	Scott J Norton <scott.norton@...com>
Subject: Re: [PATCH v11 0/4] Introducing a queue read/write lock
 implementation

On Thu, Jan 30, 2014 at 10:50:41AM -0500, Waiman Long wrote:
> One more thing, I often see line like
> 
> #define queue_write_unlock queue_write_unlock
> 
> So exactly what effect does this macro have?

Makes sure the below doesn't emit another version.

#ifndef queue_write_unlock
/**
 * queue_write_unlock - release write lock of a queue rwlock
 * @lock : Pointer to queue rwlock structure
 */
static inline void queue_write_unlock(struct qrwlock *lock)
{
	/*
	 * If the writer field is atomic, it can be cleared directly.
	 * Otherwise, an atomic subtraction will be used to clear it.
	 */
	smp_mb__before_atomic_dec();
	atomic_sub(_QW_LOCKED, &lock->cnts);
}
#endif
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