Date:	Tue, 19 Nov 2013 11:42:32 -0800
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, linux-mm <linux-mm@...ck.org>,
	linux-arch@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Waiman Long <waiman.long@...com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Alex Shi <alex.shi@...aro.org>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	George Spelvin <linux@...izon.com>,
	"H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Scott J Norton <scott.norton@...com>,
	Will Deacon <will.deacon@....com>,
	"Figo.zhang" <figo1802@...il.com>
Subject: Re: [PATCH v5 1/4] MCS Lock: Restructure the MCS lock defines and
 locking code into its own file

On Tue, 2013-11-19 at 11:10 -0800, Paul E. McKenney wrote:
> On Fri, Nov 08, 2013 at 11:51:52AM -0800, Tim Chen wrote:
> > We will need the MCS lock code for doing optimistic spinning for rwsem
> > and queue rwlock.  Extracting the MCS code from mutex.c and put into
> > its own file allow us to reuse this code easily.
> > 
> > Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
> > Signed-off-by: Davidlohr Bueso <davidlohr@...com>
> 
> Please see comments below.
> 

Thanks for reviewing the code.

> 							Thanx, Paul
> 
> > ---
> >  include/linux/mcs_spinlock.h |   64 ++++++++++++++++++++++++++++++++++++++++++
> >  include/linux/mutex.h        |    5 ++-
> >  kernel/locking/mutex.c       |   60 ++++----------------------------------
> >  3 files changed, 74 insertions(+), 55 deletions(-)
> >  create mode 100644 include/linux/mcs_spinlock.h
> > 
> > diff --git a/include/linux/mcs_spinlock.h b/include/linux/mcs_spinlock.h
> > new file mode 100644
> > index 0000000..b5de3b0
> > --- /dev/null
> > +++ b/include/linux/mcs_spinlock.h
> > @@ -0,0 +1,64 @@
> > +/*
> > + * MCS lock defines
> > + *
> > + * This file contains the main data structure and API definitions of MCS lock.
> > + *
> > + * The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spin-lock
> > + * with the desirable properties of being fair, and with each cpu trying
> > + * to acquire the lock spinning on a local variable.
> > + * It avoids expensive cache bouncings that common test-and-set spin-lock
> > + * implementations incur.
> > + */
> > +#ifndef __LINUX_MCS_SPINLOCK_H
> > +#define __LINUX_MCS_SPINLOCK_H
> > +
> > +struct mcs_spinlock {
> > +	struct mcs_spinlock *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_spinlock **lock, struct mcs_spinlock *node)
> > +{
> > +	struct mcs_spinlock *prev;
> > +
> > +	/* Init node */
> > +	node->locked = 0;
> > +	node->next   = NULL;
> > +
> > +	prev = xchg(lock, node);
> 
> OK, the full memory barriers implied by xchg() ensure that *node will be
> initialized before the "ACCESS_ONCE(prev->next) = node" below puts the
> node into the list.  This rules out the misordering scenario that Tim
> Chen called out in message-id <1380322005.3467.186.camel@...en9-DESK>
> on September 27th.
> 
> Assuming of course a corresponding barrier on the lock handoff side.
> 
> > +	if (likely(prev == NULL)) {
> > +		/* Lock acquired */
> > +		node->locked = 1;
> > +		return;
> > +	}
> > +	ACCESS_ONCE(prev->next) = node;
> > +	smp_wmb();
> 
> I don't see what the above memory barrier does.  Here are some things
> that it cannot be doing:
> 
> o	Ordering the insertion into the list above with the polling
> 	below.  First, smp_wmb() does not order prior writes against
> 	later reads, and second misordering is harmless.  If we start
> 	polling before the insertion is complete, all that happens
> 	is that the first few polls have no chance of seeing a lock
> 	grant.
> 
> o	Ordering the polling against the initialization -- the above
> 	xchg() is already doing that for us.
> 
> So what is its purpose?

Agree that the smp_wmb is not needed.  It is in the existing mcs code
residing in mutex.c and we're re-factoring the code only here and hasn't
corrected the memory barrier.

The particular smp_wmb() is removed in Patch 4/4 that corrects the
memory barriers.

> 
> > +	/* Wait until the lock holder passes the lock down */
> > +	while (!ACCESS_ONCE(node->locked))
> > +		arch_mutex_cpu_relax();
> 
> On the other hand, I don't see how we get away without a barrier here.
> As written, what prevents the caller's load from ->owner from being
> reordered with the above load from ->locked?  (Perhaps you can argue
> that such reordering is only a performance problem, but if so we need
> that argument recorded in comments.)
> 
> Of course, if anyone ever tries to use mcs_spin_lock() as a full lock,
> they will need a memory barrier here to prevent the critical section
> from leaking out.

Agree too.  The appropriate memory barrier is added in Patch 4/4.

> 
> > +}
> > +
> > +static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
> > +{
> > +	struct mcs_spinlock *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();
> > +	}
> 
> We need a memory barrier somewhere before here in this function,
> otherwise the critical section can leak out.  I do not believe that
> we can rely on the prohibition against speculative stores that Peter
> Zijlstra and I have been discussing because that does not provide the
> transitivity required by locking primitives.  I believe that we -could-
> make the access below be an smp_store_release(), though.
> 
> Placing the barrier here (or at least not preceding the initial
> fetch from node->next) has the advantage of allowing it to pair with
> the xchg() in mcs_spin_lock(), though given the dependency only an
> smp_read_barrier_depends() is required for that purpose.
> 
> > +	ACCESS_ONCE(next->locked) = 1;
> > +	smp_wmb();
> 
> I don't see what this barrier does for us.  It is ordering the unlock
> store with what, exactly?
> 
> If it really is doing something, we need a big fat comment stating what
> that is, and checkpatch.pl will be happy to inform you.  ;-)
> 
> > +}
> > +
> > +#endif /* __LINUX_MCS_SPINLOCK_H */
> > diff --git a/include/linux/mutex.h b/include/linux/mutex.h
> > index bab49da..32a32e6 100644
> > --- a/include/linux/mutex.h
> > +++ b/include/linux/mutex.h
> > @@ -46,6 +46,7 @@
> >   * - detects multi-task circular deadlocks and prints out all affected
> >   *   locks and tasks (and only those tasks)
> >   */
> > +struct mcs_spinlock;
> >  struct mutex {
> >  	/* 1: unlocked, 0: locked, negative: locked, possible waiters */
> >  	atomic_t		count;
> > @@ -55,7 +56,7 @@ struct mutex {
> >  	struct task_struct	*owner;
> >  #endif
> >  #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
> > -	void			*spin_mlock;	/* Spinner MCS lock */
> > +	struct mcs_spinlock	*mcs_lock;	/* Spinner MCS lock */
> >  #endif
> >  #ifdef CONFIG_DEBUG_MUTEXES
> >  	const char 		*name;
> > @@ -179,4 +180,4 @@ extern int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock);
> >  # define arch_mutex_cpu_relax() cpu_relax()
> >  #endif
> > 
> > -#endif
> > +#endif /* __LINUX_MUTEX_H */
> > diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> > index d24105b..e08b183 100644
> > --- a/kernel/locking/mutex.c
> > +++ b/kernel/locking/mutex.c
> > @@ -25,6 +25,7 @@
> >  #include <linux/spinlock.h>
> >  #include <linux/interrupt.h>
> >  #include <linux/debug_locks.h>
> > +#include <linux/mcs_spinlock.h>
> > 
> >  /*
> >   * In the DEBUG case we are using the "NULL fastpath" for mutexes,
> > @@ -52,7 +53,7 @@ __mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
> >  	INIT_LIST_HEAD(&lock->wait_list);
> >  	mutex_clear_owner(lock);
> >  #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
> > -	lock->spin_mlock = NULL;
> > +	lock->mcs_lock = NULL;
> >  #endif
> > 
> >  	debug_mutex_init(lock, name, key);
> > @@ -111,54 +112,7 @@ EXPORT_SYMBOL(mutex_lock);
> >   * more or less simultaneously, the spinners need to acquire a MCS lock
> >   * first before spinning on the owner field.
> >   *
> > - * We don't inline mspin_lock() so that perf can correctly account for the
> > - * time spent in this lock function.
> >   */
> > -struct mspin_node {
> > -	struct mspin_node *next ;
> > -	int		  locked;	/* 1 if lock acquired */
> > -};
> > -#define	MLOCK(mutex)	((struct mspin_node **)&((mutex)->spin_mlock))
> > -
> > -static noinline
> > -void mspin_lock(struct mspin_node **lock, struct mspin_node *node)
> > -{
> > -	struct mspin_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();
> > -	/* Wait until the lock holder passes the lock down */
> > -	while (!ACCESS_ONCE(node->locked))
> > -		arch_mutex_cpu_relax();
> > -}
> > -
> > -static void mspin_unlock(struct mspin_node **lock, struct mspin_node *node)
> > -{
> > -	struct mspin_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();
> > -}
> > 
> >  /*
> >   * Mutex spinning code migrated from kernel/sched/core.c
> > @@ -448,7 +402,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> > 
> >  	for (;;) {
> >  		struct task_struct *owner;
> > -		struct mspin_node  node;
> > +		struct mcs_spinlock  node;
> > 
> >  		if (use_ww_ctx && ww_ctx->acquired > 0) {
> >  			struct ww_mutex *ww;
> > @@ -470,10 +424,10 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> >  		 * If there's an owner, wait for it to either
> >  		 * release the lock or go to sleep.
> >  		 */
> > -		mspin_lock(MLOCK(lock), &node);
> > +		mcs_spin_lock(&lock->mcs_lock, &node);
> >  		owner = ACCESS_ONCE(lock->owner);
> >  		if (owner && !mutex_spin_on_owner(lock, owner)) {
> > -			mspin_unlock(MLOCK(lock), &node);
> > +			mcs_spin_unlock(&lock->mcs_lock, &node);
> >  			goto slowpath;
> >  		}
> > 
> > @@ -488,11 +442,11 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> >  			}
> > 
> >  			mutex_set_owner(lock);
> > -			mspin_unlock(MLOCK(lock), &node);
> > +			mcs_spin_unlock(&lock->mcs_lock, &node);
> >  			preempt_enable();
> >  			return 0;
> >  		}
> > -		mspin_unlock(MLOCK(lock), &node);
> > +		mcs_spin_unlock(&lock->mcs_lock, &node);
> > 
> >  		/*
> >  		 * When there's no owner, we might have preempted between the
> > -- 
> > 1.7.4.4
> > 
> > 
> > 
> 
> --
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