Re: [PATCH RFC] rcu: Make rcu_barrier() less disruptive

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

On Thu, Mar 15, 2012 at 01:45:27PM -0400, Mathieu Desnoyers wrote:
> * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > The rcu_barrier() primitive interrupts each and every CPU, registering
> > a callback on every CPU.  Once all of these callbacks have been invoked,
> > rcu_barrier() knows that every callback that was registered before
> > the call to rcu_barrier() has also been invoked.
> > 
> > However, there is no point in registering a callback on a CPU that
> > currently has no callbacks, most especially if that CPU is in a
> > deep idle state.  This commit therefore makes rcu_barrier() avoid
> > interrupting CPUs that have no callbacks.  Doing this requires reworking
> > the handling of orphaned callbacks, otherwise callbacks could slip through
> > rcu_barrier()'s net by being orphaned from a CPU that rcu_barrier() had
> > not yet interrupted to a CPU that rcu_barrier() had already interrupted.
> > This reworking was needed anyway to take a first step towards weaning
> > RCU from the CPU_DYING notifier's use of stop_cpu().
> 
> Quoting Documentation/RCU/rcubarrier.txt:
> 
> "We instead need the rcu_barrier() primitive. This primitive is similar
> to synchronize_rcu(), but instead of waiting solely for a grace
> period to elapse, it also waits for all outstanding RCU callbacks to
> complete. Pseudo-code using rcu_barrier() is as follows:"
> 
> The patch you propose seems like a good approach to make rcu_barrier
> less disruptive, but everyone need to be aware that rcu_barrier() would
> quit having the side-effect of doing the equivalent of
> "synchronize_rcu()" from now on: within this new approach, in the case
> where there are no pending callbacks, rcu_barrier() could, AFAIU, return
> without waiting for the current grace period to complete.
> 
> Any use of rcu_barrier() that would assume that a synchronize_rcu() is
> implicit with the rcu_barrier() execution would be a bug anyway, but
> those might only show up after this patch is applied. I would therefore
> recommend to audit all rcu_barrier() users to ensure none is expecting
> rcu_barrier to act as a synchronize_rcu before pushing this change.

Good catch!

I am going to chicken out and explicitly wait for a grace period if there
were no callbacks.  Having rcu_barrier() very rarely be a quick no-op does
sound like a standing invitation for subtle non-reproducible bugs.  ;-)

							Thanx, Paul

> Thanks,
> 
> Mathieu
> 
> > 
> > Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > 
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index 403306b..8269656 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -75,6 +75,8 @@ static struct lock_class_key rcu_node_class[NUM_RCU_LVLS];
> >  	.gpnum = -300, \
> >  	.completed = -300, \
> >  	.onofflock = __RAW_SPIN_LOCK_UNLOCKED(&structname##_state.onofflock), \
> > +	.orphan_nxttail = &structname##_state.orphan_nxtlist, \
> > +	.orphan_donetail = &structname##_state.orphan_donelist, \
> >  	.fqslock = __RAW_SPIN_LOCK_UNLOCKED(&structname##_state.fqslock), \
> >  	.n_force_qs = 0, \
> >  	.n_force_qs_ngp = 0, \
> > @@ -145,6 +147,13 @@ static void invoke_rcu_callbacks(struct rcu_state *rsp, struct rcu_data *rdp);
> >  unsigned long rcutorture_testseq;
> >  unsigned long rcutorture_vernum;
> >  
> > +/* State information for rcu_barrier() and friends. */
> > +
> > +static DEFINE_PER_CPU(struct rcu_head, rcu_barrier_head) = {NULL};
> > +static atomic_t rcu_barrier_cpu_count;
> > +static DEFINE_MUTEX(rcu_barrier_mutex);
> > +static DECLARE_WAIT_QUEUE_HEAD(rcu_barrier_wq);
> > +
> >  /*
> >   * Return true if an RCU grace period is in progress.  The ACCESS_ONCE()s
> >   * permit this function to be invoked without holding the root rcu_node
> > @@ -1311,7 +1320,60 @@ rcu_check_quiescent_state(struct rcu_state *rsp, struct rcu_data *rdp)
> >  #ifdef CONFIG_HOTPLUG_CPU
> >  
> >  /*
> > - * Move a dying CPU's RCU callbacks to online CPU's callback list.
> > + * Adopt the RCU callbacks from the specified rcu_state structure's
> > + * orphanage.  The caller must hold the ->onofflock.
> > + */
> > +static void rcu_adopt_orphan_cbs(struct rcu_state *rsp)
> > +{
> > +	int i;
> > +	struct rcu_data *rdp = __this_cpu_ptr(rsp->rda);
> > +
> > +	/*
> > +	 * If there is an rcu_barrier() operation in progress, then
> > +	 * only the task doing that operation is permitted to adopt
> > +	 * callbacks.  To do otherwise breaks rcu_barrier() and friends
> > +	 * by causing them to fail to wait for the callbacks in the
> > +	 * orphanage.
> > +	 */
> > +	if (rsp->rcu_barrier_in_progress &&
> > +	    rsp->rcu_barrier_in_progress != current)
> > +		return;
> > +
> > +	/* Do the accounting first. */
> > +	rdp->qlen_lazy += rsp->qlen_lazy;
> > +	rdp->qlen += rsp->qlen;
> > +	rdp->n_cbs_adopted += rsp->qlen;
> > +	rsp->qlen_lazy = 0;
> > +	rsp->qlen = 0;
> > +
> > +	/*
> > +	 * We do not need a memory barrier here because the only way we
> > +	 * can get here if there is an rcu_barrier() in flight is if
> > +	 * we are the task doing the rcu_barrier().
> > +	 */
> > +
> > +	/* First adopt the ready-to-invoke callbacks. */
> > +	if (rsp->orphan_donelist != NULL) {
> > +		*rsp->orphan_donetail = *rdp->nxttail[RCU_DONE_TAIL];
> > +		*rdp->nxttail[RCU_DONE_TAIL] = rsp->orphan_donelist;
> > +		for (i = RCU_NEXT_SIZE - 1; i >= RCU_DONE_TAIL; i--)
> > +			if (rdp->nxttail[i] == rdp->nxttail[RCU_DONE_TAIL])
> > +				rdp->nxttail[i] = rsp->orphan_donetail;
> > +		rsp->orphan_donelist = NULL;
> > +		rsp->orphan_donetail = &rsp->orphan_donelist;
> > +	}
> > +
> > +	/* And then adopt the callbacks that still need a grace period. */
> > +	if (rsp->orphan_nxtlist != NULL) {
> > +		*rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_nxtlist;
> > +		rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_nxttail;
> > +		rsp->orphan_nxtlist = NULL;
> > +		rsp->orphan_nxttail = &rsp->orphan_nxtlist;
> > +	}
> > +}
> > +
> > +/*
> > + * Move a dying CPU's RCU callbacks to the rcu_state structure's orphanage.
> >   * Also record a quiescent state for this CPU for the current grace period.
> >   * Synchronization and interrupt disabling are not required because
> >   * this function executes in stop_machine() context.  Therefore, cleanup
> > @@ -1325,64 +1387,67 @@ rcu_check_quiescent_state(struct rcu_state *rsp, struct rcu_data *rdp)
> >  static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
> >  {
> >  	int i;
> > +	unsigned long flags;
> >  	unsigned long mask;
> > -	int receive_cpu = cpumask_any(cpu_online_mask);
> > +	bool orphaned = 0;
> >  	struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
> > -	struct rcu_data *receive_rdp = per_cpu_ptr(rsp->rda, receive_cpu);
> >  	RCU_TRACE(struct rcu_node *rnp = rdp->mynode); /* For dying CPU. */
> >  
> > -	/* First, adjust the counts. */
> > +	/* Move the callbacks to the orphanage under ->onofflock protection. */
> > +	raw_spin_lock_irqsave(&rsp->onofflock, flags);
> > +
> > +	/* First adjust the counts. */
> >  	if (rdp->nxtlist != NULL) {
> > -		receive_rdp->qlen_lazy += rdp->qlen_lazy;
> > -		receive_rdp->qlen += rdp->qlen;
> > +		rsp->qlen_lazy += rdp->qlen_lazy;
> > +		rsp->qlen += rdp->qlen;
> > +		rdp->n_cbs_orphaned += rdp->qlen;
> >  		rdp->qlen_lazy = 0;
> >  		rdp->qlen = 0;
> > +		orphaned = 1;
> >  	}
> >  
> >  	/*
> > -	 * Next, move ready-to-invoke callbacks to be invoked on some
> > -	 * other CPU.  These will not be required to pass through another
> > -	 * grace period:  They are done, regardless of CPU.
> > +	 * Next, move those callbacks still needing a grace period to
> > +	 * the orphanage, where some other CPU will pick them up.
> > +	 * Some of the callbacks might have gone partway through a grace
> > +	 * period, but that is too bad.  They get to start over because we
> > +	 * cannot assume that grace periods are synchronized across CPUs.
> > +	 * We don't bother updating the ->nxttail[] array yet, instead
> > +	 * we just reset the whole thing later on.
> >  	 */
> > -	if (rdp->nxtlist != NULL &&
> > -	    rdp->nxttail[RCU_DONE_TAIL] != &rdp->nxtlist) {
> > -		struct rcu_head *oldhead;
> > -		struct rcu_head **oldtail;
> > -		struct rcu_head **newtail;
> > -
> > -		oldhead = rdp->nxtlist;
> > -		oldtail = receive_rdp->nxttail[RCU_DONE_TAIL];
> > -		rdp->nxtlist = *rdp->nxttail[RCU_DONE_TAIL];
> > -		*rdp->nxttail[RCU_DONE_TAIL] = *oldtail;
> > -		*receive_rdp->nxttail[RCU_DONE_TAIL] = oldhead;
> > -		newtail = rdp->nxttail[RCU_DONE_TAIL];
> > -		for (i = RCU_DONE_TAIL; i < RCU_NEXT_SIZE; i++) {
> > -			if (receive_rdp->nxttail[i] == oldtail)
> > -				receive_rdp->nxttail[i] = newtail;
> > -			if (rdp->nxttail[i] == newtail)
> > -				rdp->nxttail[i] = &rdp->nxtlist;
> > -		}
> > +	if (*rdp->nxttail[RCU_DONE_TAIL] != NULL) {
> > +		*rsp->orphan_nxttail = *rdp->nxttail[RCU_DONE_TAIL];
> > +		rsp->orphan_nxttail = rdp->nxttail[RCU_NEXT_TAIL];
> > +		*rdp->nxttail[RCU_DONE_TAIL] = NULL;
> >  	}
> >  
> >  	/*
> > -	 * Finally, put the rest of the callbacks at the end of the list.
> > -	 * The ones that made it partway through get to start over:  We
> > -	 * cannot assume that grace periods are synchronized across CPUs.
> > -	 * (We could splice RCU_WAIT_TAIL into RCU_NEXT_READY_TAIL, but
> > -	 * this does not seem compelling.  Not yet, anyway.)
> > +	 * Then move the ready-to-invoke callbacks to the orphanage,
> > +	 * where some other CPU will pick them up.  These will not be
> > +	 * required to pass though another grace period: They are done.
> >  	 */
> >  	if (rdp->nxtlist != NULL) {
> > -		*receive_rdp->nxttail[RCU_NEXT_TAIL] = rdp->nxtlist;
> > -		receive_rdp->nxttail[RCU_NEXT_TAIL] =
> > -				rdp->nxttail[RCU_NEXT_TAIL];
> > -		receive_rdp->n_cbs_adopted += rdp->qlen;
> > -		rdp->n_cbs_orphaned += rdp->qlen;
> > -
> > -		rdp->nxtlist = NULL;
> > -		for (i = 0; i < RCU_NEXT_SIZE; i++)
> > -			rdp->nxttail[i] = &rdp->nxtlist;
> > +		*rsp->orphan_donetail = rdp->nxtlist;
> > +		rsp->orphan_donetail = rdp->nxttail[RCU_DONE_TAIL];
> >  	}
> >  
> > +	/* Finally, initialize the rcu_data structure's list to empty.  */
> > +	rdp->nxtlist = NULL;
> > +	for (i = 0; i < RCU_NEXT_SIZE; i++)
> > +		rdp->nxttail[i] = &rdp->nxtlist;
> > +
> > +	/*
> > +	 * Wait up the rcu_barrier() task if there is one and if we
> > +	 * actually sent anything to the orphanage.  Except that we
> > +	 * must delay the wakeup until ->onofflock is released to
> > +	 * avoid deadlock.
> > +	 */
> > +	if (!rsp->rcu_barrier_in_progress)
> > +		orphaned = 0;
> > +	raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
> > +	if (orphaned)
> > +		wake_up(&rcu_barrier_wq);
> > +
> >  	/*
> >  	 * Record a quiescent state for the dying CPU.  This is safe
> >  	 * only because we have already cleared out the callbacks.
> > @@ -1415,11 +1480,14 @@ static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
> >  	rcu_stop_cpu_kthread(cpu);
> >  	rcu_node_kthread_setaffinity(rnp, -1);
> >  
> > -	/* Remove the dying CPU from the bitmasks in the rcu_node hierarchy. */
> > +	/* Remove the dead CPU from the bitmasks in the rcu_node hierarchy. */
> >  
> >  	/* Exclude any attempts to start a new grace period. */
> >  	raw_spin_lock_irqsave(&rsp->onofflock, flags);
> >  
> > +	/* Collect the dead CPU's callbacks. */
> > +	rcu_adopt_orphan_cbs(rsp);
> > +
> >  	/* Remove the outgoing CPU from the masks in the rcu_node hierarchy. */
> >  	mask = rdp->grpmask;	/* rnp->grplo is constant. */
> >  	do {
> > @@ -1456,6 +1524,10 @@ static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
> >  
> >  #else /* #ifdef CONFIG_HOTPLUG_CPU */
> >  
> > +static void rcu_adopt_orphan_cbs(struct rcu_state *rsp)
> > +{
> > +}
> > +
> >  static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
> >  {
> >  }
> > @@ -1524,9 +1596,6 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
> >  			    rcu_is_callbacks_kthread());
> >  
> >  	/* Update count, and requeue any remaining callbacks. */
> > -	rdp->qlen_lazy -= count_lazy;
> > -	rdp->qlen -= count;
> > -	rdp->n_cbs_invoked += count;
> >  	if (list != NULL) {
> >  		*tail = rdp->nxtlist;
> >  		rdp->nxtlist = list;
> > @@ -1536,6 +1605,10 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
> >  			else
> >  				break;
> >  	}
> > +	smp_mb(); /* List handling before counting for rcu_barrier(). */
> > +	rdp->qlen_lazy -= count_lazy;
> > +	rdp->qlen -= count;
> > +	rdp->n_cbs_invoked += count;
> >  
> >  	/* Reinstate batch limit if we have worked down the excess. */
> >  	if (rdp->blimit == LONG_MAX && rdp->qlen <= qlowmark)
> > @@ -1824,13 +1897,14 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
> >  	rdp = this_cpu_ptr(rsp->rda);
> >  
> >  	/* Add the callback to our list. */
> > -	*rdp->nxttail[RCU_NEXT_TAIL] = head;
> > -	rdp->nxttail[RCU_NEXT_TAIL] = &head->next;
> >  	rdp->qlen++;
> >  	if (lazy)
> >  		rdp->qlen_lazy++;
> >  	else
> >  		rcu_idle_count_callbacks_posted();
> > +	smp_mb();  /* Count before adding callback for rcu_barrier(). */
> > +	*rdp->nxttail[RCU_NEXT_TAIL] = head;
> > +	rdp->nxttail[RCU_NEXT_TAIL] = &head->next;
> >  
> >  	if (__is_kfree_rcu_offset((unsigned long)func))
> >  		trace_rcu_kfree_callback(rsp->name, head, (unsigned long)func,
> > @@ -2169,15 +2243,10 @@ static int rcu_cpu_has_callbacks(int cpu)
> >  	       rcu_preempt_cpu_has_callbacks(cpu);
> >  }
> >  
> > -static DEFINE_PER_CPU(struct rcu_head, rcu_barrier_head) = {NULL};
> > -static atomic_t rcu_barrier_cpu_count;
> > -static DEFINE_MUTEX(rcu_barrier_mutex);
> > -static struct completion rcu_barrier_completion;
> > -
> >  static void rcu_barrier_callback(struct rcu_head *notused)
> >  {
> >  	if (atomic_dec_and_test(&rcu_barrier_cpu_count))
> > -		complete(&rcu_barrier_completion);
> > +		wake_up(&rcu_barrier_wq);
> >  }
> >  
> >  /*
> > @@ -2203,27 +2272,121 @@ static void _rcu_barrier(struct rcu_state *rsp,
> >  			 void (*call_rcu_func)(struct rcu_head *head,
> >  					       void (*func)(struct rcu_head *head)))
> >  {
> > -	BUG_ON(in_interrupt());
> > +	int cpu;
> > +	unsigned long flags;
> > +	struct rcu_data *rdp;
> > +	struct rcu_head rh;
> > +
> > +	init_rcu_head_on_stack(&rh);
> > +
> >  	/* Take mutex to serialize concurrent rcu_barrier() requests. */
> >  	mutex_lock(&rcu_barrier_mutex);
> > -	init_completion(&rcu_barrier_completion);
> > +
> > +	smp_mb();  /* Prevent any prior operations from leaking in. */
> > +
> >  	/*
> > -	 * Initialize rcu_barrier_cpu_count to 1, then invoke
> > -	 * rcu_barrier_func() on each CPU, so that each CPU also has
> > -	 * incremented rcu_barrier_cpu_count.  Only then is it safe to
> > -	 * decrement rcu_barrier_cpu_count -- otherwise the first CPU
> > -	 * might complete its grace period before all of the other CPUs
> > -	 * did their increment, causing this function to return too
> > -	 * early.  Note that on_each_cpu() disables irqs, which prevents
> > -	 * any CPUs from coming online or going offline until each online
> > -	 * CPU has queued its RCU-barrier callback.
> > +	 * Initialize the count to one rather than to zero in order to
> > +	 * avoid a too-soon return to zero in case of a short grace period
> > +	 * (or preemption of this task).  Also flag this task as doing
> > +	 * an rcu_barrier().  This will prevent anyone else from adopting
> > +	 * orphaned callbacks, which could cause otherwise failure if a
> > +	 * CPU went offline and quickly came back online.  To see this,
> > +	 * consider the following sequence of events:
> > +	 *
> > +	 * 1.	We cause CPU 0 to post an rcu_barrier_callback() callback.
> > +	 * 2.	CPU 1 goes offline, orphaning its callbacks.
> > +	 * 3.	CPU 0 adopts CPU 1's orphaned callbacks.
> > +	 * 4.	CPU 1 comes back online.
> > +	 * 5.	We cause CPU 1 to post an rcu_barrier_callback() callback.
> > +	 * 6.	Both rcu_barrier_callback() callbacks are invoked, awakening
> > +	 *	us -- but before CPU 1's orphaned callbacks are invoked!!!
> >  	 */
> >  	atomic_set(&rcu_barrier_cpu_count, 1);
> > -	on_each_cpu(rcu_barrier_func, (void *)call_rcu_func, 1);
> > -	if (atomic_dec_and_test(&rcu_barrier_cpu_count))
> > -		complete(&rcu_barrier_completion);
> > -	wait_for_completion(&rcu_barrier_completion);
> > +	raw_spin_lock_irqsave(&rsp->onofflock, flags);
> > +	rsp->rcu_barrier_in_progress = current;
> > +	raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
> > +
> > +	/*
> > +	 * Force every CPU with callbacks to register a new callback
> > +	 * that will tell us when all the preceding callbacks have
> > +	 * been invoked.  If an offline CPU has callbacks, wait for
> > +	 * it to either come back online or to finish orphaning those
> > +	 * callbacks.
> > +	 */
> > +	for_each_possible_cpu(cpu) {
> > +		preempt_disable();
> > +		rdp = per_cpu_ptr(rsp->rda, cpu);
> > +		if (cpu_is_offline(cpu)) {
> > +			preempt_enable();
> > +			while (cpu_is_offline(cpu) &&
> > +			       ACCESS_ONCE(rdp->qlen))
> > +				schedule_timeout_interruptible(1);
> > +		} else if (ACCESS_ONCE(rdp->qlen)) {
> > +			smp_call_function_single(cpu, rcu_barrier_func,
> > +						 (void *)call_rcu_func, 1);
> > +			preempt_enable();
> > +		}
> > +	}
> > +
> > +	/*
> > +	 * Force any ongoing CPU-hotplug operations to complete,
> > +	 * so that any callbacks from the outgoing CPUs are now in
> > +	 * the orphanage.
> > +	 */
> > +	cpu_maps_update_begin();
> > +	cpu_maps_update_done();
> > +
> > +	/*
> > +	 * Now that all online CPUs have rcu_barrier_callback() callbacks
> > +	 * posted, we can adopt all of the orphaned callbacks and place
> > +	 * an rcu_barrier_callback() callback after them.  When that is done,
> > +	 * we are guaranteed to have an rcu_barrier_callback() callback
> > +	 * following every callback that could possibly have been
> > +	 * registered before _rcu_barrier() was called.
> > +	 */
> > +	raw_spin_lock_irqsave(&rsp->onofflock, flags);
> > +	rcu_adopt_orphan_cbs(rsp);
> > +	atomic_inc(&rcu_barrier_cpu_count);
> > +	call_rcu_func(&rh, rcu_barrier_callback);
> > +	raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
> > +
> > +	/*
> > +	 * Now that we have an rcu_barrier_callback() callback on each
> > +	 * CPU, and thus each counted, remove the initial count.
> > +	 */
> > +	atomic_dec(&rcu_barrier_cpu_count);
> > +	smp_mb__after_atomic_dec();
> > +
> > +	/*
> > +	 * Loop waiting for all rcu_barrier_callback() callbacks to be
> > +	 * invoked.  Adopt any orphaned callbacks in the meantime, just
> > +	 * in case one of the rcu_barrier_callback() callbacks is orphaned.
> > +	 */
> > +	while (atomic_read(&rcu_barrier_cpu_count) > 0) {
> > +		wait_event(rcu_barrier_wq,
> > +			   atomic_read(&rcu_barrier_cpu_count) == 0 ||
> > +			   ACCESS_ONCE(rsp->qlen));
> > +		if (ACCESS_ONCE(rsp->qlen)) {
> > +			raw_spin_lock_irqsave(&rsp->onofflock, flags);
> > +			rcu_adopt_orphan_cbs(rsp);
> > +			raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
> > +		}
> > +	}
> > +
> > +	/*
> > +	 * Done, so let others adopt orphaned callbacks.  But avoid
> > +	 * indefinite postponement of any additional orphans by adopting
> > +	 * one more time.
> > +	 */
> > +	raw_spin_lock_irqsave(&rsp->onofflock, flags);
> > +	rcu_adopt_orphan_cbs(rsp);
> > +	rsp->rcu_barrier_in_progress = NULL;
> > +	raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
> > +
> > +	/* Other rcu_barrier() invocations can now safely proceed. */
> >  	mutex_unlock(&rcu_barrier_mutex);
> > +
> > +	destroy_rcu_head_on_stack(&rh);
> >  }
> >  
> >  /**
> > diff --git a/kernel/rcutree.h b/kernel/rcutree.h
> > index 36ca28e..1e49c56 100644
> > --- a/kernel/rcutree.h
> > +++ b/kernel/rcutree.h
> > @@ -371,6 +371,17 @@ struct rcu_state {
> >  
> >  	raw_spinlock_t onofflock;		/* exclude on/offline and */
> >  						/*  starting new GP. */
> > +	struct rcu_head *orphan_nxtlist;	/* Orphaned callbacks that */
> > +						/*  need a grace period. */
> > +	struct rcu_head **orphan_nxttail;	/* Tail of above. */
> > +	struct rcu_head *orphan_donelist;	/* Orphaned callbacks that */
> > +						/*  are ready to invoke. */
> > +	struct rcu_head **orphan_donetail;	/* Tail of above. */
> > +	long qlen_lazy;				/* Number of lazy callbacks. */
> > +	long qlen;				/* Total number of callbacks. */
> > +	struct task_struct *rcu_barrier_in_progress;
> > +						/* Task doing rcu_barrier(), */
> > +						/*  or NULL if no barrier. */
> >  	raw_spinlock_t fqslock;			/* Only one task forcing */
> >  						/*  quiescent states. */
> >  	unsigned long jiffies_force_qs;		/* Time at which to invoke */
> > diff --git a/kernel/rcutree_trace.c b/kernel/rcutree_trace.c
> > index ed459ed..d4bc16d 100644
> > --- a/kernel/rcutree_trace.c
> > +++ b/kernel/rcutree_trace.c
> > @@ -271,13 +271,13 @@ static void print_one_rcu_state(struct seq_file *m, struct rcu_state *rsp)
> >  
> >  	gpnum = rsp->gpnum;
> >  	seq_printf(m, "c=%lu g=%lu s=%d jfq=%ld j=%x "
> > -		      "nfqs=%lu/nfqsng=%lu(%lu) fqlh=%lu\n",
> > +		      "nfqs=%lu/nfqsng=%lu(%lu) fqlh=%lu oqlen=%ld/%ld\n",
> >  		   rsp->completed, gpnum, rsp->fqs_state,
> >  		   (long)(rsp->jiffies_force_qs - jiffies),
> >  		   (int)(jiffies & 0xffff),
> >  		   rsp->n_force_qs, rsp->n_force_qs_ngp,
> >  		   rsp->n_force_qs - rsp->n_force_qs_ngp,
> > -		   rsp->n_force_qs_lh);
> > +		   rsp->n_force_qs_lh, rsp->qlen_lazy, rsp->qlen);
> >  	for (rnp = &rsp->node[0]; rnp - &rsp->node[0] < NUM_RCU_NODES; rnp++) {
> >  		if (rnp->level != level) {
> >  			seq_puts(m, "\n");
> > 
> 
> -- 
> Mathieu Desnoyers
> Operating System Efficiency R&D Consultant
> EfficiOS Inc.
> http://www.efficios.com
>