Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines andlocking code into its own file

[Ingo Molnar]

* Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> wrote:

> We will need the MCS lock code for doing optimistic spinning for rwsem.
> Extracting the MCS code from mutex.c and put into its own file allow us
> to reuse this code easily for rwsem.
> 
> Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Signed-off-by: Davidlohr Bueso <davidlohr@xxxxxx>
> ---
>  include/linux/mcslock.h |   58 +++++++++++++++++++++++++++++++++++++++++++++++
>  kernel/mutex.c          |   58 +++++-----------------------------------------
>  2 files changed, 65 insertions(+), 51 deletions(-)
>  create mode 100644 include/linux/mcslock.h
> 
> diff --git a/include/linux/mcslock.h b/include/linux/mcslock.h
> new file mode 100644
> index 0000000..20fd3f0
> --- /dev/null
> +++ b/include/linux/mcslock.h
> @@ -0,0 +1,58 @@
> +/*
> + * MCS lock defines
> + *
> + * This file contains the main data structure and API definitions of MCS lock.

A (very) short blurb about what an MCS lock is would be nice here.

> + */
> +#ifndef __LINUX_MCSLOCK_H
> +#define __LINUX_MCSLOCK_H
> +
> +struct mcs_spin_node {
> +	struct mcs_spin_node *next;
> +	int		  locked;	/* 1 if lock acquired */
> +};

The vertical alignment looks broken here.

> +
> +/*
> + * We don't inline mcs_spin_lock() so that perf can correctly account for the
> + * time spent in this lock function.
> + */
> +static noinline
> +void mcs_spin_lock(struct mcs_spin_node **lock, struct mcs_spin_node *node)
> +{
> +	struct mcs_spin_node *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();
> +}
> +
> +static void mcs_spin_unlock(struct mcs_spin_node **lock, struct mcs_spin_node *node)
> +{
> +	struct mcs_spin_node *next = ACCESS_ONCE(node->next);
> +
> +	if (likely(!next)) {
> +		/*
> +		 * Release the lock by setting it to NULL
> +		 */
> +		if (cmpxchg(lock, node, NULL) == node)
> +			return;
> +		/* Wait until the next pointer is set */
> +		while (!(next = ACCESS_ONCE(node->next)))
> +			arch_mutex_cpu_relax();
> +	}
> +	ACCESS_ONCE(next->locked) = 1;
> +	smp_wmb();
> +}
> +
> +#endif

We typically close header guards not via a plain #endif but like this:

#endif /* __LINUX_SPINLOCK_H */

#endif /* __LINUX_SPINLOCK_TYPES_H */

etc.

Thanks,

	Ingo
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/