linux-kernel - Re: [PATCH v2] rwsem: Support optimistic spinning

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Message-ID: <20140430082715.GA11096@twins.programming.kicks-ass.net>
Date:	Wed, 30 Apr 2014 10:27:15 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Alex Shi <alex.shi@...aro.org>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E.McKenney" <paulmck@...ux.vnet.ibm.com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] rwsem: Support optimistic spinning

On Mon, Apr 28, 2014 at 03:09:01PM -0700, Davidlohr Bueso wrote:
> +/*
> + * Try to acquire write lock before the writer has been put on wait queue.
> + */
> +static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
> +{
> +	long count = ACCESS_ONCE(sem->count);
> +retry:
> +	if (count == RWSEM_WAITING_BIAS) {
> +		count = cmpxchg(&sem->count, RWSEM_WAITING_BIAS,
> +			    RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS);
> +		/* allow write lock stealing, try acquiring the write lock. */
> +		if (count == RWSEM_WAITING_BIAS)
> +			goto acquired;
> +		else if (count == 0)
> +			goto retry;
> +	} else if (count == 0) {
> +		count = cmpxchg(&sem->count, 0, RWSEM_ACTIVE_WRITE_BIAS);
> +		if (count == 0)
> +			goto acquired;
> +		else if (count == RWSEM_WAITING_BIAS)
> +			goto retry;
> +	}
> +	return false;
> +
> +acquired:
> +	return true;
> +}

Could we have written that like:

static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
{
	long old, count = ACCESS_ONCE(sem->count);

	for (;;) {
		if (!(count == 0 || count == RWSEM_WAITING_BIAS))
			return false;

		old = cmpxchg(&sem->count, count, count + RWSEM_ACTIVE_BIAS);
		if (old == count)
			return true;

		count = old;
	}
}

?

Just checking my brain got all the branches untangled right :-)
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