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

Paul E. McKenney
2013-09-27T20:39:10Z

On Fri, Sep 27, 2013 at 12:38:53PM -0700, Tim Chen wrote:
> 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.

I don't immediately see the need for the smp_wmb() in either case.

> Tim
>
> > > + /* Wait until the lock holder passes the lock down */
> > > + while (!ACCESS_ONCE(node->locked))
> > > + arch_mutex_cpu_relax();

However, you do need a full memory barrier here in order to ensure that
you see the effects of the previous lock holder's critical section.

Thanx, Paul

> > > +}
> > > +
> > > +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
> >
>
>