tick: fix tick_broadcast_pending_mask not clearedtimers/fix/core

The recent modification in the cpuidle framework consolidated the timer
broadcast code across the different drivers by setting a new flag in the idle
state. It tells the cpuidle core code to enter/exit to the broadcast mode for
the cpu when entering a deep idle state. The broadcast timer enter/exit is no
longer handled by the back-end driver.

This change made the local interrupt to be enabled *before* calling
CLOCK_EVENT_NOTIFY_EXIT. bad or not (see below) ?

On a tegra114, a four cores system, when the flag has been introduced in the
driver, the following warning appeared:

[   25.629559] WARNING: CPU: 2 PID: 0 at
kernel/time/tick-broadcast.c:578 tick_broadcast_oneshot_control
+0x1a4/0x1d0()
[   25.629565] Modules linked in:
[   25.629574] CPU: 2 PID: 0 Comm: swapper/2 Not tainted
3.10.0-rc3-next-20130529+ #15
[   25.629601] [<c00148f4>] (unwind_backtrace+0x0/0xf8) from
[<c0011040>] (show_stack+0x10/0x14)
[   25.629616] [<c0011040>] (show_stack+0x10/0x14) from [<c0504248>]
(dump_stack+0x80/0xc4)
[   25.629633] [<c0504248>] (dump_stack+0x80/0xc4) from [<c00231ac>]
(warn_slowpath_common+0x64/0x88)
[   25.629646] [<c00231ac>] (warn_slowpath_common+0x64/0x88) from
[<c00231ec>] (warn_slowpath_null+0x1c/0x24)
[   25.629657] [<c00231ec>] (warn_slowpath_null+0x1c/0x24) from
[<c00667f8>] (tick_broadcast_oneshot_control+0x1a4/0x1d0)
[   25.629670] [<c00667f8>] (tick_broadcast_oneshot_control+0x1a4/0x1d0)
from [<c0065cd0>] (tick_notify+0x240/0x40c)
[   25.629685] [<c0065cd0>] (tick_notify+0x240/0x40c) from [<c0044724>]
(notifier_call_chain+0x44/0x84)
[   25.629699] [<c0044724>] (notifier_call_chain+0x44/0x84) from
[<c0044828>] (raw_notifier_call_chain+0x18/0x20)
[   25.629712] [<c0044828>] (raw_notifier_call_chain+0x18/0x20) from
[<c00650cc>] (clockevents_notify+0x28/0x170)
[   25.629726] [<c00650cc>] (clockevents_notify+0x28/0x170) from
[<c033f1f0>] (cpuidle_idle_call+0x11c/0x168)
[   25.629739] [<c033f1f0>] (cpuidle_idle_call+0x11c/0x168) from
[<c000ea94>] (arch_cpu_idle+0x8/0x38)
[   25.629755] [<c000ea94>] (arch_cpu_idle+0x8/0x38) from [<c005ea80>]
(cpu_startup_entry+0x60/0x134)
[   25.629767] [<c005ea80>] (cpu_startup_entry+0x60/0x134) from
[<804fe9a4>] (0x804fe9a4)
[   25.629772] ---[ end trace 5484e77e2531bccc ]---

I don't have the hardware, so I wasn't able to reproduce the warning but after
looking a while in the code, I deduced the following:

 1. the CPU2 enters a deep idle state and sets the broadcast timer
 2. the timer expires, the tick_handle_oneshot_broadcast function is called,
    setting the tick_broadcast_pending_mask and waking up the idle cpu CPU2
 3. the CPU2 exits idle and invokes tick_broadcast_oneshot_control with
    CLOCK_EVENT_NOTIFY_EXIT with the following code:
    [...]
    if (dev->next_event.tv64 == KTIME_MAX)
            goto out;

    if (cpumask_test_and_clear_cpu(cpu,
                                 tick_broadcast_pending_mask))
            goto out;
    [...]

 4. if there is no next event planned for CPU2, we fulfil the first condition and
    we jump to the 'out' section without clearing the tick_broadcast_pending_mask

 5. CPU2 goes to deep idle again and calls tick_broadcast_oneshot_control with
    CLOCK_NOTIFY_EVENT_ENTER but with the tick_broadcast_pending_mask set for
    CPU2, leading to the WARNING.

Above, it is mentionned the change moved the CLOCK_EVENT_NOTIFY_EXIT after the
local interrupt were enabled. If it is before, this warning does not occur. My
hypothesis is the code path described before does not happen because when a
broadcast timer expires, dev->next_event.tv64 is always different from KTIME_MAX
because the timer handler did not set the value yet (local interrupt are still
disabled).

I don't see anywhere in the code, a clockevents_notify(ENTER/EXIT) block must be
done with the local interrupt disabled in between, furthermore the function uses
'raw_spin_lock_irqsave' which make me think, we don't care about that.

Invert the conditions and make the tick broadcast code immune from the local
interrupts context.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reported-by: Joseph Lo <josephl@nvidia.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
index d067c01586f5..58d88e828d38 100644
--- a/kernel/time/tick-broadcast.c
+++ b/kernel/time/tick-broadcast.c
@@ -610,8 +610,6 @@ void tick_broadcast_oneshot_control(unsigned long reason)
 	} else {
 		if (cpumask_test_and_clear_cpu(cpu, tick_broadcast_oneshot_mask)) {
 			clockevents_set_mode(dev, CLOCK_EVT_MODE_ONESHOT);
-			if (dev->next_event.tv64 == KTIME_MAX)
-				goto out;
 			/*
 			 * The cpu which was handling the broadcast
 			 * timer marked this cpu in the broadcast
@@ -625,6 +623,8 @@ void tick_broadcast_oneshot_control(unsigned long reason)
 				       tick_broadcast_pending_mask))
 				goto out;
 
+			if (dev->next_event.tv64 == KTIME_MAX)
+				goto out;
 			/*
 			 * If the pending bit is not set, then we are
 			 * either the CPU handling the broadcast