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    Subject[tip: sched/core] rcu/nocb: Perform deferred wake up before last idle's need_resched() check
    The following commit has been merged into the sched/core branch of tip:

    Commit-ID: 43789ef3f7d61aa7bed0cb2764e588fc990c30ef
    Gitweb: https://git.kernel.org/tip/43789ef3f7d61aa7bed0cb2764e588fc990c30ef
    Author: Frederic Weisbecker <frederic@kernel.org>
    AuthorDate: Mon, 01 Feb 2021 00:05:45 +01:00
    Committer: Ingo Molnar <mingo@kernel.org>
    CommitterDate: Wed, 17 Feb 2021 14:12:43 +01:00

    rcu/nocb: Perform deferred wake up before last idle's need_resched() check

    Entering RCU idle mode may cause a deferred wake up of an RCU NOCB_GP
    kthread (rcuog) to be serviced.

    Usually a local wake up happening while running the idle task is handled
    in one of the need_resched() checks carefully placed within the idle
    loop that can break to the scheduler.

    Unfortunately the call to rcu_idle_enter() is already beyond the last
    generic need_resched() check and we may halt the CPU with a resched
    request unhandled, leaving the task hanging.

    Fix this with splitting the rcuog wakeup handling from rcu_idle_enter()
    and place it before the last generic need_resched() check in the idle
    loop. It is then assumed that no call to call_rcu() will be performed
    after that in the idle loop until the CPU is put in low power mode.

    Fixes: 96d3fd0d315a (rcu: Break call_rcu() deadlock involving scheduler and perf)
    Reported-by: Paul E. McKenney <paulmck@kernel.org>
    Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Cc: stable@vger.kernel.org
    Link: https://lkml.kernel.org/r/20210131230548.32970-3-frederic@kernel.org
    ---
    include/linux/rcupdate.h | 2 ++
    kernel/rcu/tree.c | 3 ---
    kernel/rcu/tree_plugin.h | 5 +++++
    kernel/sched/idle.c | 1 +
    4 files changed, 8 insertions(+), 3 deletions(-)

    diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
    index fd02c5f..36c2119 100644
    --- a/include/linux/rcupdate.h
    +++ b/include/linux/rcupdate.h
    @@ -110,8 +110,10 @@ static inline void rcu_user_exit(void) { }

    #ifdef CONFIG_RCU_NOCB_CPU
    void rcu_init_nohz(void);
    +void rcu_nocb_flush_deferred_wakeup(void);
    #else /* #ifdef CONFIG_RCU_NOCB_CPU */
    static inline void rcu_init_nohz(void) { }
    +static inline void rcu_nocb_flush_deferred_wakeup(void) { }
    #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */

    /**
    diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
    index 63032e5..82838e9 100644
    --- a/kernel/rcu/tree.c
    +++ b/kernel/rcu/tree.c
    @@ -671,10 +671,7 @@ static noinstr void rcu_eqs_enter(bool user)
    */
    void rcu_idle_enter(void)
    {
    - struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
    -
    lockdep_assert_irqs_disabled();
    - do_nocb_deferred_wakeup(rdp);
    rcu_eqs_enter(false);
    }
    EXPORT_SYMBOL_GPL(rcu_idle_enter);
    diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
    index 7e291ce..d5b38c2 100644
    --- a/kernel/rcu/tree_plugin.h
    +++ b/kernel/rcu/tree_plugin.h
    @@ -2187,6 +2187,11 @@ static void do_nocb_deferred_wakeup(struct rcu_data *rdp)
    do_nocb_deferred_wakeup_common(rdp);
    }

    +void rcu_nocb_flush_deferred_wakeup(void)
    +{
    + do_nocb_deferred_wakeup(this_cpu_ptr(&rcu_data));
    +}
    +
    void __init rcu_init_nohz(void)
    {
    int cpu;
    diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
    index 305727e..7199e6f 100644
    --- a/kernel/sched/idle.c
    +++ b/kernel/sched/idle.c
    @@ -285,6 +285,7 @@ static void do_idle(void)
    }

    arch_cpu_idle_enter();
    + rcu_nocb_flush_deferred_wakeup();

    /*
    * In poll mode we reenable interrupts and spin. Also if we
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