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    Subject[PATCH v4 tip/core/rcu 29/38] rcu-tasks: Disable CPU hotplug across RCU tasks trace scans
    Date
    From: "Paul E. McKenney" <paulmck@kernel.org>

    This commit disables CPU hotplug across RCU tasks trace scans, which
    is a first step towards correctly recognizing idle tasks "running" on
    offline CPUs.

    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
    ---
    kernel/rcu/tasks.h | 18 ++++++++++++++----
    1 file changed, 14 insertions(+), 4 deletions(-)

    diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
    index 0cdeeb3..e8c03bd 100644
    --- a/kernel/rcu/tasks.h
    +++ b/kernel/rcu/tasks.h
    @@ -906,16 +906,16 @@ static void rcu_tasks_trace_pregp_step(void)
    {
    int cpu;

    - // Wait for CPU-hotplug paths to complete.
    - cpus_read_lock();
    - cpus_read_unlock();
    -
    // Allow for fast-acting IPIs.
    atomic_set(&trc_n_readers_need_end, 1);

    // There shouldn't be any old IPIs, but...
    for_each_possible_cpu(cpu)
    WARN_ON_ONCE(per_cpu(trc_ipi_to_cpu, cpu));
    +
    + // Disable CPU hotplug across the tasklist scan.
    + // This also waits for all readers in CPU-hotplug code paths.
    + cpus_read_lock();
    }

    /* Do first-round processing for the specified task. */
    @@ -931,6 +931,9 @@ static void rcu_tasks_trace_pertask(struct task_struct *t,
    /* Do intermediate processing between task and holdout scans. */
    static void rcu_tasks_trace_postscan(void)
    {
    + // Re-enable CPU hotplug now that the tasklist scan has completed.
    + cpus_read_unlock();
    +
    // Wait for late-stage exiting tasks to finish exiting.
    // These might have passed the call to exit_tasks_rcu_finish().
    synchronize_rcu();
    @@ -975,6 +978,9 @@ static void check_all_holdout_tasks_trace(struct list_head *hop,
    {
    struct task_struct *g, *t;

    + // Disable CPU hotplug across the holdout list scan.
    + cpus_read_lock();
    +
    list_for_each_entry_safe(t, g, hop, trc_holdout_list) {
    // If safe and needed, try to check the current task.
    if (READ_ONCE(t->trc_ipi_to_cpu) == -1 &&
    @@ -987,6 +993,10 @@ static void check_all_holdout_tasks_trace(struct list_head *hop,
    else if (needreport)
    show_stalled_task_trace(t, firstreport);
    }
    +
    + // Re-enable CPU hotplug now that the holdout list scan has completed.
    + cpus_read_unlock();
    +
    if (needreport) {
    if (firstreport)
    pr_err("INFO: rcu_tasks_trace detected stalls? (Late IPI?)\n");
    --
    2.9.5
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