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    Subject[PATCH 4/4] rcu: Fix early call to rcu_irq_exit()
    Date
    On the irq exit path, tick_nohz_stop_sched_tick()
    may raise a softirq, which action leads to the wake up
    path and select_task_rq_fair() that makes use of rcu
    to iterate the domains.

    This is an illegal use of RCU because we may be in dyntick
    idle mode:

    [ 132.978883] ===============================
    [ 132.978883] [ INFO: suspicious RCU usage. ]
    [ 132.978883] -------------------------------
    [ 132.978883] kernel/sched_fair.c:1707 suspicious rcu_dereference_check() usage!
    [ 132.978883]
    [ 132.978883] other info that might help us debug this:
    [ 132.978883]
    [ 132.978883]
    [ 132.978883] rcu_scheduler_active = 1, debug_locks = 0
    [ 132.978883] RCU used illegally from extended quiescent state!
    [ 132.978883] 2 locks held by swapper/0:
    [ 132.978883] #0: (&p->pi_lock){-.-.-.}, at: [<ffffffff8105a729>] try_to_wake_up+0x39/0x2f0
    [ 132.978883] #1: (rcu_read_lock){.+.+..}, at: [<ffffffff8105556a>] select_task_rq_fair+0x6a/0xec0
    [ 132.978883]
    [ 132.978883] stack backtrace:
    [ 132.978883] Pid: 0, comm: swapper Tainted: G W 3.0.0+ #178
    [ 132.978883] Call Trace:
    [ 132.978883] <IRQ> [<ffffffff810a01f6>] lockdep_rcu_suspicious+0xe6/0x100
    [ 132.978883] [<ffffffff81055c49>] select_task_rq_fair+0x749/0xec0
    [ 132.978883] [<ffffffff8105556a>] ? select_task_rq_fair+0x6a/0xec0
    [ 132.978883] [<ffffffff812fe494>] ? do_raw_spin_lock+0x54/0x150
    [ 132.978883] [<ffffffff810a1f2d>] ? trace_hardirqs_on+0xd/0x10
    [ 132.978883] [<ffffffff8105a7c3>] try_to_wake_up+0xd3/0x2f0
    [ 132.978883] [<ffffffff81094f98>] ? ktime_get+0x68/0xf0
    [ 132.978883] [<ffffffff8105aa35>] wake_up_process+0x15/0x20
    [ 132.978883] [<ffffffff81069dd5>] raise_softirq_irqoff+0x65/0x110
    [ 132.978883] [<ffffffff8108eb65>] __hrtimer_start_range_ns+0x415/0x5a0
    [ 132.978883] [<ffffffff812fe3ee>] ? do_raw_spin_unlock+0x5e/0xb0
    [ 132.978883] [<ffffffff8108ed08>] hrtimer_start+0x18/0x20
    [ 132.978883] [<ffffffff8109c9c3>] tick_nohz_stop_sched_tick+0x393/0x450
    [ 132.978883] [<ffffffff810694f2>] irq_exit+0xd2/0x100
    [ 132.978883] [<ffffffff81829e96>] do_IRQ+0x66/0xe0
    [ 132.978883] [<ffffffff81820d53>] common_interrupt+0x13/0x13
    [ 132.978883] <EOI> [<ffffffff8103434b>] ? native_safe_halt+0xb/0x10
    [ 132.978883] [<ffffffff810a1f2d>] ? trace_hardirqs_on+0xd/0x10
    [ 132.978883] [<ffffffff810144ea>] default_idle+0xba/0x370
    [ 132.978883] [<ffffffff810147fe>] amd_e400_idle+0x5e/0x130
    [ 132.978883] [<ffffffff8100a9f6>] cpu_idle+0xb6/0x120
    [ 132.978883] [<ffffffff817f217f>] rest_init+0xef/0x150
    [ 132.978883] [<ffffffff817f20e2>] ? rest_init+0x52/0x150
    [ 132.978883] [<ffffffff81ed9cf3>] start_kernel+0x3da/0x3e5
    [ 132.978883] [<ffffffff81ed9346>] x86_64_start_reservations+0x131/0x135
    [ 132.978883] [<ffffffff81ed944d>] x86_64_start_kernel+0x103/0x112

    Fix this by calling rcu_irq_exit() after tick_nohz_stop_sched_tick().

    Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
    Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Ingo Molnar <mingo@elte.hu>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    ---
    kernel/softirq.c | 2 +-
    1 files changed, 1 insertions(+), 1 deletions(-)

    diff --git a/kernel/softirq.c b/kernel/softirq.c
    index fca82c3..d4047b8 100644
    --- a/kernel/softirq.c
    +++ b/kernel/softirq.c
    @@ -347,12 +347,12 @@ void irq_exit(void)
    if (!in_interrupt() && local_softirq_pending())
    invoke_softirq();

    - rcu_irq_exit();
    #ifdef CONFIG_NO_HZ
    /* Make sure that timer wheel updates are propagated */
    if (idle_cpu(smp_processor_id()) && !in_interrupt() && !need_resched())
    tick_nohz_stop_sched_tick(0);
    #endif
    + rcu_irq_exit();
    preempt_enable_no_resched();
    }

    --
    1.7.5.4


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