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Commit 1331e7a1 authored by Paul E. McKenney's avatar Paul E. McKenney Committed by Paul E. McKenney
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rcu: Remove _rcu_barrier() dependency on __stop_machine() 



Currently, _rcu_barrier() relies on preempt_disable() to prevent
any CPU from going offline, which in turn depends on CPU hotplug's
use of __stop_machine().

This patch therefore makes _rcu_barrier() use get_online_cpus() to
block CPU-hotplug operations.  This has the added benefit of removing
the need for _rcu_barrier() to adopt callbacks:  Because CPU-hotplug
operations are excluded, there can be no callbacks to adopt.  This
commit simplifies the code accordingly.

Signed-off-by: default avatarPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: default avatarJosh Triplett <josh@joshtriplett.org>
parent a10d206e

kernel/rcutree.c

+11 −72
Original line number Diff line number Diff line
@@ -1392,17 +1392,6 @@ 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;
@@ -1457,9 +1446,8 @@ static void rcu_cleanup_dying_cpu(struct rcu_state *rsp) 
 * The CPU has been completely removed, and some other CPU is reporting

 * this fact from process context.  Do the remainder of the cleanup,

 * including orphaning the outgoing CPU's RCU callbacks, and also

 * adopting them, if there is no _rcu_barrier() instance running.

 * There can only be one CPU hotplug operation at a time, so no other

 * CPU can be attempting to update rcu_cpu_kthread_task.

 * adopting them.  There can only be one CPU hotplug operation at a time,

 * so no other CPU can be attempting to update rcu_cpu_kthread_task.

 */

static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)

{
@@ -1521,10 +1509,6 @@ 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)

{

}
@@ -2328,13 +2312,10 @@ static void rcu_barrier_func(void *type) 
static void _rcu_barrier(struct rcu_state *rsp)

{

	int cpu;

	unsigned long flags;

	struct rcu_data *rdp;

	struct rcu_data rd;

	unsigned long snap = ACCESS_ONCE(rsp->n_barrier_done);

	unsigned long snap_done;



	init_rcu_head_on_stack(&rd.barrier_head);

	_rcu_barrier_trace(rsp, "Begin", -1, snap);



	/* Take mutex to serialize concurrent rcu_barrier() requests. */
@@ -2374,70 +2355,30 @@ static void _rcu_barrier(struct rcu_state *rsp) 
	/*

	 * 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!!!

	 * (or preemption of this task).  Exclude CPU-hotplug operations

	 * to ensure that no offline CPU has callbacks queued.

	 */

	init_completion(&rsp->barrier_completion);

	atomic_set(&rsp->barrier_cpu_count, 1);

	raw_spin_lock_irqsave(&rsp->onofflock, flags);

	rsp->rcu_barrier_in_progress = current;

	raw_spin_unlock_irqrestore(&rsp->onofflock, flags);

	get_online_cpus();



	/*

	 * 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.

	 * Force each CPU with callbacks to register a new callback.

	 * When that callback is invoked, we will know that all of the

	 * corresponding CPU's preceding callbacks have been invoked.

	 */

	for_each_possible_cpu(cpu) {

		preempt_disable();

	for_each_online_cpu(cpu) {

		rdp = per_cpu_ptr(rsp->rda, cpu);

		if (cpu_is_offline(cpu)) {

			_rcu_barrier_trace(rsp, "Offline", cpu,

					   rsp->n_barrier_done);

			preempt_enable();

			while (cpu_is_offline(cpu) && ACCESS_ONCE(rdp->qlen))

				schedule_timeout_interruptible(1);

		} else if (ACCESS_ONCE(rdp->qlen)) {

		if (ACCESS_ONCE(rdp->qlen)) {

			_rcu_barrier_trace(rsp, "OnlineQ", cpu,

					   rsp->n_barrier_done);

			smp_call_function_single(cpu, rcu_barrier_func, rsp, 1);

			preempt_enable();

		} else {

			_rcu_barrier_trace(rsp, "OnlineNQ", cpu,

					   rsp->n_barrier_done);

			preempt_enable();

		}

	}



	/*

	 * 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);

	rsp->rcu_barrier_in_progress = NULL;

	raw_spin_unlock_irqrestore(&rsp->onofflock, flags);

	atomic_inc(&rsp->barrier_cpu_count);

	smp_mb__after_atomic_inc(); /* Ensure atomic_inc() before callback. */

	rd.rsp = rsp;

	rsp->call(&rd.barrier_head, rcu_barrier_callback);

	put_online_cpus();



	/*

	 * Now that we have an rcu_barrier_callback() callback on each
@@ -2458,8 +2399,6 @@ static void _rcu_barrier(struct rcu_state *rsp) 


	/* Other rcu_barrier() invocations can now safely proceed. */

	mutex_unlock(&rsp->barrier_mutex);



	destroy_rcu_head_on_stack(&rd.barrier_head);

}



/**

kernel/rcutree.h

+0 −3
Original line number Diff line number Diff line
@@ -398,9 +398,6 @@ struct rcu_state { 
	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. */

	struct mutex barrier_mutex;		/* Guards barrier fields. */

	atomic_t barrier_cpu_count;		/* # CPUs waiting on. */

	struct completion barrier_completion;	/* Wake at barrier end. */

kernel/rcutree_trace.c

+2 −2
Original line number Diff line number Diff line
@@ -51,8 +51,8 @@ static int show_rcubarrier(struct seq_file *m, void *unused) 
	struct rcu_state *rsp;



	for_each_rcu_flavor(rsp)

		seq_printf(m, "%s: %c bcc: %d nbd: %lu\n",

			   rsp->name, rsp->rcu_barrier_in_progress ? 'B' : '.',

		seq_printf(m, "%s: bcc: %d nbd: %lu\n",

			   rsp->name,

			   atomic_read(&rsp->barrier_cpu_count),

			   rsp->n_barrier_done);

	return 0;