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Commit e81baf4c authored by Sebastian Andrzej Siewior's avatar Sebastian Andrzej Siewior Committed by Paul E. McKenney
Browse files

srcu: Remove srcu_queue_delayed_work_on() 



srcu_queue_delayed_work_on() disables preemption (and therefore CPU
hotplug in RCU's case) and then checks based on its own accounting if a
CPU is online. If the CPU is online it uses queue_delayed_work_on()
otherwise it fallbacks to queue_delayed_work().
The problem here is that queue_work() on -RT does not work with disabled
preemption.

queue_work_on() works also on an offlined CPU. queue_delayed_work_on()
has the problem that it is possible to program a timer on an offlined
CPU. This timer will fire once the CPU is online again. But until then,
the timer remains programmed and nothing will happen.

Add a local timer which will fire (as requested per delay) on the local
CPU and then enqueue the work on the specific CPU.

RCUtorture testing with SRCU-P for 24h showed no problems.

Signed-off-by: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.ibm.com>
parent c8ca1aa7

include/linux/srcutree.h

+2 −1
Original line number Diff line number Diff line
@@ -45,7 +45,8 @@ struct srcu_data { 
	unsigned long srcu_gp_seq_needed;	/* Furthest future GP needed. */

	unsigned long srcu_gp_seq_needed_exp;	/* Furthest future exp GP. */

	bool srcu_cblist_invoking;		/* Invoking these CBs? */

	struct delayed_work work;		/* Context for CB invoking. */

	struct timer_list delay_work;		/* Delay for CB invoking */

	struct work_struct work;		/* Context for CB invoking. */

	struct rcu_head srcu_barrier_head;	/* For srcu_barrier() use. */

	struct srcu_node *mynode;		/* Leaf srcu_node. */

	unsigned long grpmask;			/* Mask for leaf srcu_node */

kernel/rcu/srcutree.c

+24 −31
Original line number Diff line number Diff line
@@ -58,6 +58,7 @@ static bool __read_mostly srcu_init_done; 
static void srcu_invoke_callbacks(struct work_struct *work);

static void srcu_reschedule(struct srcu_struct *ssp, unsigned long delay);

static void process_srcu(struct work_struct *work);

static void srcu_delay_timer(struct timer_list *t);



/* Wrappers for lock acquisition and release, see raw_spin_lock_rcu_node(). */

#define spin_lock_rcu_node(p)					\
@@ -156,7 +157,8 @@ static void init_srcu_struct_nodes(struct srcu_struct *ssp, bool is_static) 
			snp->grphi = cpu;

		}

		sdp->cpu = cpu;

		INIT_DELAYED_WORK(&sdp->work, srcu_invoke_callbacks);

		INIT_WORK(&sdp->work, srcu_invoke_callbacks);

		timer_setup(&sdp->delay_work, srcu_delay_timer, 0);

		sdp->ssp = ssp;

		sdp->grpmask = 1 << (cpu - sdp->mynode->grplo);

		if (is_static)
@@ -386,12 +388,18 @@ void _cleanup_srcu_struct(struct srcu_struct *ssp, bool quiesced) 
	} else {

		flush_delayed_work(&ssp->work);

	}

	for_each_possible_cpu(cpu)

	for_each_possible_cpu(cpu) {

		struct srcu_data *sdp = per_cpu_ptr(ssp->sda, cpu);



		if (quiesced) {

			if (WARN_ON(delayed_work_pending(&per_cpu_ptr(ssp->sda, cpu)->work)))

			if (WARN_ON(timer_pending(&sdp->delay_work)))

				return; /* Just leak it! */

			if (WARN_ON(work_pending(&sdp->work)))

				return; /* Just leak it! */

		} else {

			flush_delayed_work(&per_cpu_ptr(ssp->sda, cpu)->work);

			del_timer_sync(&sdp->delay_work);

			flush_work(&sdp->work);

		}

	}

	if (WARN_ON(rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)) != SRCU_STATE_IDLE) ||

	    WARN_ON(srcu_readers_active(ssp))) {
@@ -463,39 +471,23 @@ static void srcu_gp_start(struct srcu_struct *ssp) 
	WARN_ON_ONCE(state != SRCU_STATE_SCAN1);

}



/*

 * Track online CPUs to guide callback workqueue placement.

 */

DEFINE_PER_CPU(bool, srcu_online);



void srcu_online_cpu(unsigned int cpu)

static void srcu_delay_timer(struct timer_list *t)

{

	WRITE_ONCE(per_cpu(srcu_online, cpu), true);

}

	struct srcu_data *sdp = container_of(t, struct srcu_data, delay_work);



void srcu_offline_cpu(unsigned int cpu)

{

	WRITE_ONCE(per_cpu(srcu_online, cpu), false);

	queue_work_on(sdp->cpu, rcu_gp_wq, &sdp->work);

}



/*

 * Place the workqueue handler on the specified CPU if online, otherwise

 * just run it whereever.  This is useful for placing workqueue handlers

 * that are to invoke the specified CPU's callbacks.

 */

static bool srcu_queue_delayed_work_on(int cpu, struct workqueue_struct *wq,

				       struct delayed_work *dwork,

static void srcu_queue_delayed_work_on(struct srcu_data *sdp,

				       unsigned long delay)

{

	bool ret;

	if (!delay) {

		queue_work_on(sdp->cpu, rcu_gp_wq, &sdp->work);

		return;

	}



	preempt_disable();

	if (READ_ONCE(per_cpu(srcu_online, cpu)))

		ret = queue_delayed_work_on(cpu, wq, dwork, delay);

	else

		ret = queue_delayed_work(wq, dwork, delay);

	preempt_enable();

	return ret;

	timer_reduce(&sdp->delay_work, jiffies + delay);

}



/*
@@ -504,7 +496,7 @@ static bool srcu_queue_delayed_work_on(int cpu, struct workqueue_struct *wq, 
 */

static void srcu_schedule_cbs_sdp(struct srcu_data *sdp, unsigned long delay)

{

	srcu_queue_delayed_work_on(sdp->cpu, rcu_gp_wq, &sdp->work, delay);

	srcu_queue_delayed_work_on(sdp, delay);

}



/*
@@ -1186,7 +1178,8 @@ static void srcu_invoke_callbacks(struct work_struct *work) 
	struct srcu_data *sdp;

	struct srcu_struct *ssp;



	sdp = container_of(work, struct srcu_data, work.work);

	sdp = container_of(work, struct srcu_data, work);



	ssp = sdp->ssp;

	rcu_cblist_init(&ready_cbs);

	spin_lock_irq_rcu_node(sdp);

kernel/rcu/tree.c

+0 −4
Original line number Diff line number Diff line
@@ -3408,8 +3408,6 @@ int rcutree_online_cpu(unsigned int cpu) 
	raw_spin_lock_irqsave_rcu_node(rnp, flags);

	rnp->ffmask |= rdp->grpmask;

	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);

	if (IS_ENABLED(CONFIG_TREE_SRCU))

		srcu_online_cpu(cpu);

	if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)

		return 0; /* Too early in boot for scheduler work. */

	sync_sched_exp_online_cleanup(cpu);
@@ -3434,8 +3432,6 @@ int rcutree_offline_cpu(unsigned int cpu) 
	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);



	rcutree_affinity_setting(cpu, cpu);

	if (IS_ENABLED(CONFIG_TREE_SRCU))

		srcu_offline_cpu(cpu);

	return 0;

}

kernel/rcu/tree.h

+0 −8
Original line number Diff line number Diff line
@@ -458,11 +458,3 @@ static void rcu_bind_gp_kthread(void); 
static bool rcu_nohz_full_cpu(void);

static void rcu_dynticks_task_enter(void);

static void rcu_dynticks_task_exit(void);
 


#ifdef CONFIG_SRCU

void srcu_online_cpu(unsigned int cpu);

void srcu_offline_cpu(unsigned int cpu);

#else /* #ifdef CONFIG_SRCU */

void srcu_online_cpu(unsigned int cpu) { }

void srcu_offline_cpu(unsigned int cpu) { }

#endif /* #else #ifdef CONFIG_SRCU */