linux-kernel - Re: [PATCH v9 1/5] qrwlock: A queue read/write lock implementation

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Date:	Mon, 20 Jan 2014 16:21:29 +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 Chandramouleeswaran\"" <aswin@...com>,
	Scott J Norton <scott.norton@...com>
Subject: Re: [PATCH v9 1/5] qrwlock: A queue read/write lock implementation

On Tue, Jan 14, 2014 at 11:44:03PM -0500, Waiman Long wrote:
> +#ifndef arch_mutex_cpu_relax
> +# define arch_mutex_cpu_relax() cpu_relax()
> +#endif

Include <linux/mutex.h>


> +#ifndef smp_load_acquire
> +# ifdef CONFIG_X86
> +#   define smp_load_acquire(p)				\
> +	({						\
> +		typeof(*p) ___p1 = ACCESS_ONCE(*p);	\
> +		barrier();				\
> +		___p1;					\
> +	})
> +# else
> +#   define smp_load_acquire(p)				\
> +	({						\
> +		typeof(*p) ___p1 = ACCESS_ONCE(*p);	\
> +		smp_mb();				\
> +		___p1;					\
> +	})
> +# endif
> +#endif
> +
> +#ifndef smp_store_release
> +# ifdef CONFIG_X86
> +#   define smp_store_release(p, v)			\
> +	do {						\
> +		barrier();				\
> +		ACCESS_ONCE(*p) = v;			\
> +	} while (0)
> +# else
> +#   define smp_store_release(p, v)			\
> +	do {						\
> +		smp_mb();				\
> +		ACCESS_ONCE(*p) = v;			\
> +	} while (0)
> +# endif
> +#endif

Remove these.

> +/*
> + * If an xadd (exchange-add) macro isn't available, simulate one with
> + * the atomic_add_return() function.
> + */
> +#ifdef xadd
> +# define qrw_xadd(rw, inc)	xadd(&(rw).rwc, inc)
> +#else
> +# define qrw_xadd(rw, inc)	(u32)(atomic_add_return(inc, &(rw).rwa) - inc)
> +#endif

Is GCC really so stupid that you cannot always use the
atomic_add_return()? The x86 atomic_add_return is i + xadd(), so you'll
end up with:

 i + xadd() - i

Surely it can just remove the two i terms?

> +/**
> + * wait_in_queue - Add to queue and wait until it is at the head
> + * @lock: Pointer to queue rwlock structure
> + * @node: Node pointer to be added to the queue
> + */
> +static inline void wait_in_queue(struct qrwlock *lock, struct qrwnode *node)
> +{
> +	struct qrwnode *prev;
> +
> +	node->next = NULL;
> +	node->wait = true;
> +	prev = xchg(&lock->waitq, node);
> +	if (prev) {
> +		prev->next = node;
> +		/*
> +		 * Wait until the waiting flag is off
> +		 */
> +		while (smp_load_acquire(&node->wait))
> +			arch_mutex_cpu_relax();
> +	}
> +}

Please rebase on top of the MCS lock patches such that this is gone.
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