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    Subject[PATCH rcu 06/18] rcu/nocb: Make rcu_core() callbacks acceleration (de-)offloading safe
    Date
    From: Frederic Weisbecker <frederic@kernel.org>

    When callbacks are offloaded, the NOCB kthreads handle the callbacks
    progression on behalf of rcu_core().

    However during the (de-)offloading process, the kthread may not be
    entirely up to the task. As a result some callbacks grace period
    sequence number may remain stale for a while because rcu_core() won't
    take care of them either.

    Fix this with forcing callbacks acceleration from rcu_core() as long
    as the offloading process isn't complete.

    Reported-and-tested-by: Valentin Schneider <valentin.schneider@arm.com>
    Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
    Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
    Cc: Valentin Schneider <valentin.schneider@arm.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
    Cc: Josh Triplett <josh@joshtriplett.org>
    Cc: Joel Fernandes <joel@joelfernandes.org>
    Cc: Boqun Feng <boqun.feng@gmail.com>
    Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
    Cc: Uladzislau Rezki <urezki@gmail.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
    ---
    kernel/rcu/tree.c | 18 ++++++++++++++++--
    1 file changed, 16 insertions(+), 2 deletions(-)

    diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
    index 5985698f33414..cb9abb80377fe 100644
    --- a/kernel/rcu/tree.c
    +++ b/kernel/rcu/tree.c
    @@ -2278,6 +2278,7 @@ rcu_report_qs_rdp(struct rcu_data *rdp)
    unsigned long flags;
    unsigned long mask;
    bool needwake = false;
    + bool needacc = false;
    struct rcu_node *rnp;

    WARN_ON_ONCE(rdp->cpu != smp_processor_id());
    @@ -2305,16 +2306,29 @@ rcu_report_qs_rdp(struct rcu_data *rdp)
    * This GP can't end until cpu checks in, so all of our
    * callbacks can be processed during the next GP.
    *
    - * NOCB kthreads have their own way to deal with that.
    + * NOCB kthreads have their own way to deal with that...
    */
    - if (!rcu_rdp_is_offloaded(rdp))
    + if (!rcu_rdp_is_offloaded(rdp)) {
    needwake = rcu_accelerate_cbs(rnp, rdp);
    + } else if (!rcu_segcblist_completely_offloaded(&rdp->cblist)) {
    + /*
    + * ...but NOCB kthreads may miss or delay callbacks acceleration
    + * if in the middle of a (de-)offloading process.
    + */
    + needacc = true;
    + }

    rcu_disable_urgency_upon_qs(rdp);
    rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
    /* ^^^ Released rnp->lock */
    if (needwake)
    rcu_gp_kthread_wake();
    +
    + if (needacc) {
    + rcu_nocb_lock_irqsave(rdp, flags);
    + rcu_accelerate_cbs_unlocked(rnp, rdp);
    + rcu_nocb_unlock_irqrestore(rdp, flags);
    + }
    }
    }

    --
    2.31.1.189.g2e36527f23
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