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

Davidlohr Bueso
2013-09-26T20:06:54Z

On Thu, 2013-09-26 at 12:27 -0700, Jason Low wrote:
> 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?

I don't follow, the whole idea is to spin on the local variable.