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

Jason Low
2013-09-26T21:10:00Z

On Thu, 2013-09-26 at 13:40 -0700, Davidlohr Bueso wrote:
> On Thu, 2013-09-26 at 13:23 -0700, Jason Low wrote:
> > On Thu, 2013-09-26 at 13:06 -0700, Davidlohr Bueso wrote:
> > > 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.
> >
> > If prev == NULL, doesn't that mean it won't proceed to spin on the
> > variable because the lock is already free and we call return? In that
> > case where we directly acquire the lock, I was wondering if it is
> > necessary to set node->locked = 1.
>
> Yes, that's true, but we need to flag the lock as acquired (the node's
> lock is initially set to unlocked), otherwise others trying to acquire
> the lock can spin forever:
>
> /* Wait until the lock holder passes the lock down */
> while (!ACCESS_ONCE(node->locked))
> arch_mutex_cpu_relax();
>
> The ->locked variable in this implementation refers to if the lock is
> acquired, and *not* to if busy-waiting is necessary.

hmm, others threads acquiring the lock will be spinning on their own
local nodes, not this node's node->locked. And if prev == NULL, the
current thread won't be reading it's node->lock either since we return.
So what other thread is going to be reading this node's node->lock?

Thanks,
Jason