Re: [PATCH 4/4] MCS Lock: Make mcs_spinlock.h includable in other files

[Will Deacon <will.deacon@arm.com>]

Hello,

On Mon, Nov 04, 2013 at 11:37:13PM +0000, Tim Chen wrote:
> The following changes are made to enable mcs_spinlock.h file to be
> widely included in other files without causing problem:
> 
> 1) Include a number of prerequisite header files and define
>    arch_mutex_cpu_relax(), if not previously defined.
> 2) Separate out mcs_spin_lock() into a mcs_spinlock.c file.
> 3) Make mcs_spin_unlock() an inlined function.

[...]

> +void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
> +{
> +	struct mcs_spinlock *prev;
> +
> +	/* Init node */
> +	node->locked = 0;
> +	node->next   = NULL;
> +
> +	prev = xchg(lock, node);
> +	if (likely(prev == NULL)) {
> +		/* Lock acquired */
> +		node->locked = 1;
> +		return;
> +	}
> +	ACCESS_ONCE(prev->next) = node;
> +	smp_wmb();
> +	/* Wait until the lock holder passes the lock down */
> +	while (!ACCESS_ONCE(node->locked))
> +		arch_mutex_cpu_relax();
> +}

You have the barrier in a different place than the version in the header
file; is this intentional?

Also, why is an smp_wmb() sufficient (as opposed to a full smp_mb()?). Are
there restrictions on the types of access that can occur in the critical
section?

Will

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