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    Subject[PATCH 4.18 001/145] rcu: Make expedited GPs handle CPU 0 being offline
    Date
    4.18-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Boqun Feng <boqun.feng@gmail.com>

    commit fcc63543650150629c8a873cbef3578770acecd9 upstream.

    Currently, the parallelized initialization of expedited grace periods uses
    the workqueue associated with each rcu_node structure's ->grplo field.
    This works fine unless that CPU is offline. This commit therefore uses
    the CPU corresponding to the lowest-numbered online CPU, or just queues
    the work on WORK_CPU_UNBOUND if there are no online CPUs corresponding
    to this rcu_node structure.

    Note that this patch uses cpu_is_offline() instead of the usual approach
    of checking bits in the rcu_node structure's ->qsmaskinitnext field. This
    is safe because preemption is disabled across both the cpu_is_offline()
    check and the call to queue_work_on().

    Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
    [ paulmck: Disable preemption to close offline race window. ]
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    [ paulmck: Apply Peter Zijlstra feedback on CPU selection. ]
    Tested-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    kernel/rcu/tree_exp.h | 9 ++++++++-
    1 file changed, 8 insertions(+), 1 deletion(-)

    --- a/kernel/rcu/tree_exp.h
    +++ b/kernel/rcu/tree_exp.h
    @@ -472,6 +472,7 @@ retry_ipi:
    static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
    smp_call_func_t func)
    {
    + int cpu;
    struct rcu_node *rnp;

    trace_rcu_exp_grace_period(rsp->name, rcu_exp_gp_seq_endval(rsp), TPS("reset"));
    @@ -492,7 +493,13 @@ static void sync_rcu_exp_select_cpus(str
    continue;
    }
    INIT_WORK(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
    - queue_work_on(rnp->grplo, rcu_par_gp_wq, &rnp->rew.rew_work);
    + preempt_disable();
    + cpu = cpumask_next(rnp->grplo - 1, cpu_online_mask);
    + /* If all offline, queue the work on an unbound CPU. */
    + if (unlikely(cpu > rnp->grphi))
    + cpu = WORK_CPU_UNBOUND;
    + queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work);
    + preempt_enable();
    rnp->exp_need_flush = true;
    }


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