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    Subjectsrcu: use cpu_online() instead custom check
    > The current check via srcu_online is slightly racy because after looking
    > at srcu_online there could be an interrupt that interrupted us long
    > enough until the CPU we checked against went offline.

    I don't see how this can happen, even in -rt. The call to
    srcu_offline_cpu() happens very early in the CPU removal process,
    which means that the synchronize_rcu_mult(call_rcu, call_rcu_sched)
    in sched_cpu_deactivate() would wait for the interrupt to complete.
    And for the enclosing preempt_disable region to complete.

    Or is getting rid of that preempt_disable region the real reason for
    this change?

    > An alternative would be to hold the hotplug rwsem (so the CPUs don't
    > change their state) and then check based on cpu_online() if we queue it
    > on a specific CPU or not. queue_work_on() itself can handle if something
    > is enqueued on an offline CPU but a timer which is enqueued on an offline
    > CPU won't fire until the CPU is back online.
    >
    > I am not sure if the removal in rcu_init() is okay or not. I assume that
    > SRCU won't enqueue a work item before SRCU is up and ready.

    That was the case before the current merge window, but use of call_srcu()
    by tracing means that SRCU needs to be able to deal with call_srcu()
    long before any initialization has happened. The actual callbacks
    won't be invoked until much later, after the scheduler and workqueues
    are completely up and running, but call_srcu() can be invoked very early.

    But I am not seeing any removal in rcu_init() in this patch, so I might
    be missing something.

    Thanx, Paul

    > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
    >
    > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
    > index 6c9866a854b1..3428a40a813e 100644
    > --- a/kernel/rcu/srcutree.c
    > +++ b/kernel/rcu/srcutree.c
    > @@ -38,6 +38,7 @@
    > #include <linux/delay.h>
    > #include <linux/module.h>
    > #include <linux/srcu.h>
    > +#include <linux/cpu.h>
    >
    > #include "rcu.h"
    > #include "rcu_segcblist.h"
    > @@ -458,21 +459,6 @@ static void srcu_gp_start(struct srcu_struct *sp)
    > 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)
    > -{
    > - WRITE_ONCE(per_cpu(srcu_online, cpu), true);
    > -}
    > -
    > -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
    > @@ -484,12 +470,12 @@ static bool srcu_queue_delayed_work_on(int cpu, struct workqueue_struct *wq,
    > {
    > bool ret;
    >
    > - preempt_disable();
    > - if (READ_ONCE(per_cpu(srcu_online, cpu)))
    > + cpus_read_lock();
    > + if (cpu_online(cpu))
    > ret = queue_delayed_work_on(cpu, wq, dwork, delay);
    > else
    > ret = queue_delayed_work(wq, dwork, delay);
    > - preempt_enable();
    > + cpus_read_unlock();
    > return ret;
    > }
    >
    > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
    > index 6868ef417e9f..e2e68250009b 100644
    > --- a/kernel/rcu/tree.c
    > +++ b/kernel/rcu/tree.c
    > @@ -3767,8 +3767,6 @@ int rcutree_online_cpu(unsigned int cpu)
    > 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);
    > @@ -3796,8 +3794,6 @@ int rcutree_offline_cpu(unsigned int cpu)
    > }
    >
    > rcutree_affinity_setting(cpu, cpu);
    > - if (IS_ENABLED(CONFIG_TREE_SRCU))
    > - srcu_offline_cpu(cpu);
    > return 0;
    > }
    >

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