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

Tim Chen
2013-09-27T22:46:53Z

On Fri, 2013-09-27 at 13:38 -0700, Paul E. McKenney wrote:
> 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.

Thinking a bit more, the following could happen in Jason's
initial patch proposal. In this case variable "prev" referenced
by CPU1 points to "node" referenced by CPU2

CPU 1 (calling lock) CPU 2 (calling unlock)
ACCESS_ONCE(prev->next) = node
*next = ACCESS_ONCE(node->next);
ACCESS_ONCE(next->locked) = 1;
node->locked = 0;

Then we will be spinning forever on CPU1 as we overwrite the lock passed
from CPU2 before we check it. The original code assign
"node->locked = 0" before xchg does not have this issue.
Doing the following change of moving smp_wmb immediately
after node->locked assignment (suggested by Jason)

node->locked = 0;
smp_wmb();
ACCESS_ONCE(prev->next) = node;

could avoid the problem, but will need closer scrutiny to see if
there are other pitfalls if wmb happen before

ACCESS_ONCE(prev->next) = node;

> >
> > > > + /* 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.

Is it necessary to add a memory barrier after acquiring
the lock if the previous lock holder execute smp_wmb before passing
the lock?

Thanks.

Tim