timers: handle HRTIMER_CB_IRQSAFE_UNLOCKED correctly from softirq context

Impact: fix incorrect locking triggered during hotplug-intense stress-tests

While migrating the the CB_IRQSAFE_UNLOCKED timers during a cpu-offline,
we queue them on the cb_pending list, so that they won't go
stale.

Thus, when the callbacks of the timers run from the softirq context,
they could run into potential deadlocks, since these callbacks
assume that they're running with irq's disabled, thereby annoying
lockdep!

Fix this by emulating hardirq context while running these callbacks from
the hrtimer softirq.

=================================
[ INFO: inconsistent lock state ]
2.6.27 #2
--------------------------------
inconsistent {in-hardirq-W} -> {hardirq-on-W} usage.
ksoftirqd/0/4 [HC0[0]:SC1[1]:HE1:SE0] takes:
(&rq->lock){++..}, at: [<c011db84>] sched_rt_period_timer+0x9e/0x1fc
{in-hardirq-W} state was registered at:
[<c014103c>] __lock_acquire+0x549/0x121e
[<c0107890>] native_sched_clock+0x88/0x99
[<c013aa12>] clocksource_get_next+0x39/0x3f
[<c0139abc>] update_wall_time+0x616/0x7df
[<c0141d6b>] lock_acquire+0x5a/0x74
[<c0121724>] scheduler_tick+0x3a/0x18d
[<c047ed45>] _spin_lock+0x1c/0x45
[<c0121724>] scheduler_tick+0x3a/0x18d
[<c0121724>] scheduler_tick+0x3a/0x18d
[<c012c436>] update_process_times+0x3a/0x44
[<c013c044>] tick_periodic+0x63/0x6d
[<c013c062>] tick_handle_periodic+0x14/0x5e
[<c010568c>] timer_interrupt+0x44/0x4a
[<c0150c9f>] handle_IRQ_event+0x13/0x3d
[<c0151c14>] handle_level_irq+0x79/0xbd
[<c0105634>] do_IRQ+0x69/0x7d
[<c01041e4>] common_interrupt+0x28/0x30
[<c047007b>] aac_probe_one+0x1a3/0x3f3
[<c047ec2d>] _spin_unlock_irqrestore+0x36/0x39
[<c01512b4>] setup_irq+0x1be/0x1f9
[<c065d70b>] start_kernel+0x259/0x2c5
[<ffffffff>] 0xffffffff
irq event stamp: 50102
hardirqs last enabled at (50102): [<c047ebf4>] _spin_unlock_irq+0x20/0x23
hardirqs last disabled at (50101): [<c047edc2>] _spin_lock_irq+0xa/0x4b
softirqs last enabled at (50088): [<c0128ba6>] do_softirq+0x37/0x4d
softirqs last disabled at (50099): [<c0128ba6>] do_softirq+0x37/0x4d

other info that might help us debug this:
no locks held by ksoftirqd/0/4.

stack backtrace:
Pid: 4, comm: ksoftirqd/0 Not tainted 2.6.27 #2
[<c013f6cb>] print_usage_bug+0x13e/0x147
[<c013fef5>] mark_lock+0x493/0x797
[<c01410b1>] __lock_acquire+0x5be/0x121e
[<c0141d6b>] lock_acquire+0x5a/0x74
[<c011db84>] sched_rt_period_timer+0x9e/0x1fc
[<c047ed45>] _spin_lock+0x1c/0x45
[<c011db84>] sched_rt_period_timer+0x9e/0x1fc
[<c011db84>] sched_rt_period_timer+0x9e/0x1fc
[<c01210fd>] finish_task_switch+0x41/0xbd
[<c0107890>] native_sched_clock+0x88/0x99
[<c011dae6>] sched_rt_period_timer+0x0/0x1fc
[<c0136dda>] run_hrtimer_pending+0x54/0xe5
[<c011dae6>] sched_rt_period_timer+0x0/0x1fc
[<c0128afb>] __do_softirq+0x7b/0xef
[<c0128ba6>] do_softirq+0x37/0x4d
[<c0128c12>] ksoftirqd+0x56/0xc5
[<c0128bbc>] ksoftirqd+0x0/0xc5
[<c0134649>] kthread+0x38/0x5d
[<c0134611>] kthread+0x0/0x5d
[<c0104477>] kernel_thread_helper+0x7/0x10
=======================

Signed-off-by: Gautham R Shenoy <[e-mail]>
Acked-by: Peter Zijlstra <[e-mail]>
Acked-by: "Paul E. McKenney" <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

kernel/hrtimer.c

···
 		enum hrtimer_restart (*fn)(struct hrtimer *);
 		struct hrtimer *timer;
 		int restart;
+		int emulate_hardirq_ctx = 0;
 
 		timer = list_entry(cpu_base->cb_pending.next,
 				   struct hrtimer, cb_entry);
···
 		timer_stats_account_hrtimer(timer);
 
 		fn = timer->function;
+		/*
+		 * A timer might have been added to the cb_pending list
+		 * when it was migrated during a cpu-offline operation.
+		 * Emulate hardirq context for such timers.
+		 */
+		if (timer->cb_mode == HRTIMER_CB_IRQSAFE_PERCPU ||
+		    timer->cb_mode == HRTIMER_CB_IRQSAFE_UNLOCKED)
+			emulate_hardirq_ctx = 1;
+
 		__remove_hrtimer(timer, timer->base, HRTIMER_STATE_CALLBACK, 0);
 		spin_unlock_irq(&cpu_base->lock);
 
-		restart = fn(timer);
+		if (unlikely(emulate_hardirq_ctx)) {
+			local_irq_disable();
+			restart = fn(timer);
+			local_irq_enable();
+		} else
+			restart = fn(timer);
 
 		spin_lock_irq(&cpu_base->lock);