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

Jason Low
2013-09-26T19:27:26Z

On Wed, Sep 25, 2013 at 3:10 PM, Tim Chen <tim.c.chen@linux.intel.com> 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@linux.intel.com>
> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
> ---
> 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.
> + */
> +#ifndef __LINUX_MCSLOCK_H
> +#define __LINUX_MCSLOCK_H
> +
> +struct mcs_spin_node {
> + struct mcs_spin_node *next;
> + int locked; /* 1 if lock acquired */
> +};
> +
> +/*
> + * 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;

If we don't spin on the local node, is it necessary to set this variable?

> + 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)

And can we make this check likely()?