Re: [PATCH v5 1/4] MCS Lock: Restructure the MCS lock defines and locking code into its own file

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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@linux.intel.com>
> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>

Please see comments below.

							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@schen9-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?

> +	/* 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.

> +}
> +
> +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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