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    SubjectRe: [PATCH] srcu: Remove srcu_queue_delayed_work_on()
    On Tue, Dec 11, 2018 at 12:12:38PM +0100, Sebastian Andrzej Siewior wrote:
    > 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: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

    Queued and pushed, thank you!

    Thanx, Paul

    > ---
    > include/linux/srcutree.h | 3 ++-
    > kernel/rcu/srcutree.c | 57 ++++++++++++++++++----------------------
    > kernel/rcu/tree.c | 4 ---
    > kernel/rcu/tree.h | 8 ------
    > 4 files changed, 27 insertions(+), 45 deletions(-)
    >
    > diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
    > index 6f292bd3e7db7..0faa978c98807 100644
    > --- a/include/linux/srcutree.h
    > +++ b/include/linux/srcutree.h
    > @@ -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 */
    > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
    > index 3600d88d8956b..7f041f2435df9 100644
    > --- a/kernel/rcu/srcutree.c
    > +++ b/kernel/rcu/srcutree.c
    > @@ -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)
    > @@ -386,13 +388,19 @@ 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))) {
    > pr_info("%s: Active srcu_struct %p state: %d\n",
    > @@ -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);
    > +
    > + queue_work_on(sdp->cpu, rcu_gp_wq, &sdp->work);
    > }
    >
    > -void srcu_offline_cpu(unsigned int cpu)
    > -{
    > - WRITE_ONCE(per_cpu(srcu_online, cpu), false);
    > -}
    > -
    > -/*
    > - * 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);
    > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
    > index be67a1bcba1da..86538c72cae90 100644
    > --- a/kernel/rcu/tree.c
    > +++ b/kernel/rcu/tree.c
    > @@ -3361,8 +3361,6 @@ 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);
    > @@ -3387,8 +3385,6 @@ 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;
    > }
    >
    > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
    > index 0ab060c8e9a71..e27fb27e087df 100644
    > --- a/kernel/rcu/tree.h
    > +++ b/kernel/rcu/tree.h
    > @@ -456,11 +456,3 @@ 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 */
    > --
    > 2.20.0.rc2
    >

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