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

Tim Chen
2013-09-27T19:38:59Z

On Fri, 2013-09-27 at 08:29 -0700, Paul E. McKenney wrote:
> On Wed, Sep 25, 2013 at 03:10:49PM -0700, Tim Chen 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;
> > + return;
> > + }
> > + ACCESS_ONCE(prev->next) = node;
> > + smp_wmb();

BTW, is the above memory barrier necessary? It seems like the xchg
instruction already provided a memory barrier.

Now if we made the changes that Jason suggested:

/* Init node */
- node->locked = 0;
node->next = NULL;

prev = xchg(lock, node);
if (likely(prev == NULL)) {
/* Lock acquired */
- node->locked = 1;
return;
}
+ node->locked = 0;
ACCESS_ONCE(prev->next) = node;
smp_wmb();

We are probably still okay as other cpus do not read the value of
node->locked, which is a local variable.

Tim

> > + /* 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();
>
> Shouldn't the memory barrier precede the "ACCESS_ONCE(next->locked) = 1;"?
> Maybe in an "else" clause of the prior "if" statement, given that the
> cmpxchg() does it otherwise.
>
> Otherwise, in the case where the "if" conditionn is false, the critical
> section could bleed out past the unlock.
>
> Thanx, Paul
>