[PATCH v2] clocksource: exynos_mct: fix for sleeping in atomic ctx handling cpu hotplug notif.

[Damian Eppel]

This is to fix an issue of sleeping in atomic context when processing
hotplug notifications in Exynos MCT(Multi-Core Timer).
The issue was reproducible on Exynos 3250 (Rinato board) and Exynos 5420
(Arndale Octa board).

Whilst testing cpu hotplug events on kernel configured with DEBUG_PREEMPT
and DEBUG_ATOMIC_SLEEP we get following BUG message, caused by calling
request_irq() and free_irq() in the context of hotplug notification
(which is in this case atomic context).

root@target:~# echo 0 > /sys/devices/system/cpu/cpu1/online

[   25.157867] IRQ18 no longer affine to CPU1
...
[   25.158445] CPU1: shutdown

root@target:~# echo 1 > /sys/devices/system/cpu/cpu1/online

[   40.785859] CPU1: Software reset
[   40.786660] BUG: sleeping function called from invalid context at mm/slub.c:1241
[   40.786668] in_atomic(): 1, irqs_disabled(): 128, pid: 0, name: swapper/1
[   40.786678] Preemption disabled at:[<  (null)>]   (null)
[   40.786681]
[   40.786692] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 3.19.0-rc4-00024-g7dca860 #36
[   40.786698] Hardware name: SAMSUNG EXYNOS (Flattened Device Tree)
[   40.786728] [<c0014a00>] (unwind_backtrace) from [<c0011980>] (show_stack+0x10/0x14)
[   40.786747] [<c0011980>] (show_stack) from [<c0449ba0>] (dump_stack+0x70/0xbc)
[   40.786767] [<c0449ba0>] (dump_stack) from [<c00c6124>] (kmem_cache_alloc+0xd8/0x170)
[   40.786785] [<c00c6124>] (kmem_cache_alloc) from [<c005d6f8>] (request_threaded_irq+0x64/0x128)
[   40.786804] [<c005d6f8>] (request_threaded_irq) from [<c0350b8c>] (exynos4_local_timer_setup+0xc0/0x13c)
[   40.786820] [<c0350b8c>] (exynos4_local_timer_setup) from [<c0350ca8>] (exynos4_mct_cpu_notify+0x30/0xa8)
[   40.786838] [<c0350ca8>] (exynos4_mct_cpu_notify) from [<c003b330>] (notifier_call_chain+0x44/0x84)
[   40.786857] [<c003b330>] (notifier_call_chain) from [<c0022fd4>] (__cpu_notify+0x28/0x44)
[   40.786873] [<c0022fd4>] (__cpu_notify) from [<c0013714>] (secondary_start_kernel+0xec/0x150)
[   40.786886] [<c0013714>] (secondary_start_kernel) from [<40008764>] (0x40008764)

Solution:
Clockevent irqs cannot be requested/freed every time cpu is
hot-plugged/unplugged as CPU_STARTING/CPU_DYING notifications
that signals hotplug or unplug events are sent with both preemption
and local interrupts disabled. Since request_irq() may sleep it is
moved to the initialization stage and performed for all possible
cpus prior putting them online. Then, in the case of hotplug event
the irq asociated with the given cpu will simply be enabled.
Similarly on cpu unplug event the interrupt is not freed but just
disabled.

Note that after successful request_irq() call for a clockevent device
associated to given cpu the requested irq is disabled (via disable_irq()).
That is to make things symmetric as we expect hotplug event as a next
thing (which will enable irq again). This should not pose any problems
because at the time of requesting irq the clockevent device is not
fully initialized yet, therefore should not produce interrupts at that point.

For disabling an irq at cpu unplug notification disable_irq_nosync() is
chosen which is a non-blocking function. This again shouldn't be a problem as
prior sending CPU_DYING notification interrupts are migrated away
from the cpu.

Fixes: 7114cd749a12 ("clocksource: exynos_mct: use (request/free)_irq calls for local timer registration")
Signed-off-by: Damian Eppel <d.eppel@xxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Reported-by: Krzysztof Kozlowski <k.kozlowski@xxxxxxxxxxx>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@xxxxxxxxxxx>
Tested-by: Krzysztof Kozlowski <k.kozlowski@xxxxxxxxxxx>
(Tested on Arndale Octa Board)
Tested-by: Marcin Jabrzyk <m.jabrzyk@xxxxxxxxxxx>
(Tested on Rinato B2 (Exynos 3250) board)
---

Notes:
    Changes since v1:
            o added Krzysztof's and Marcin's Reviewed-by / Tested-by
              with information about the test HW.

 drivers/clocksource/exynos_mct.c | 45 ++++++++++++++++++++++++++++------------
 1 file changed, 32 insertions(+), 13 deletions(-)

diff --git a/drivers/clocksource/exynos_mct.c b/drivers/clocksource/exynos_mct.c
index 83564c9..9beca58 100644
--- a/drivers/clocksource/exynos_mct.c
+++ b/drivers/clocksource/exynos_mct.c
@@ -466,15 +466,12 @@ static int exynos4_local_timer_setup(struct clock_event_device *evt)
 	exynos4_mct_write(TICK_BASE_CNT, mevt->base + MCT_L_TCNTB_OFFSET);
 
 	if (mct_int_type == MCT_INT_SPI) {
-		evt->irq = mct_irqs[MCT_L0_IRQ + cpu];
-		if (request_irq(evt->irq, exynos4_mct_tick_isr,
-				IRQF_TIMER | IRQF_NOBALANCING,
-				evt->name, mevt)) {
-			pr_err("exynos-mct: cannot register IRQ %d\n",
-				evt->irq);
+
+		if (evt->irq == -1)
 			return -EIO;
-		}
-		irq_force_affinity(mct_irqs[MCT_L0_IRQ + cpu], cpumask_of(cpu));
+
+		irq_force_affinity(evt->irq, cpumask_of(cpu));
+		enable_irq(evt->irq);
 	} else {
 		enable_percpu_irq(mct_irqs[MCT_L0_IRQ], 0);
 	}
@@ -487,10 +484,12 @@ static int exynos4_local_timer_setup(struct clock_event_device *evt)
 static void exynos4_local_timer_stop(struct clock_event_device *evt)
 {
 	evt->set_mode(CLOCK_EVT_MODE_UNUSED, evt);
-	if (mct_int_type == MCT_INT_SPI)
-		free_irq(evt->irq, this_cpu_ptr(&percpu_mct_tick));
-	else
+	if (mct_int_type == MCT_INT_SPI) {
+		if (evt->irq != -1)
+			disable_irq_nosync(evt->irq);
+	} else {
 		disable_percpu_irq(mct_irqs[MCT_L0_IRQ]);
+	}
 }
 
 static int exynos4_mct_cpu_notify(struct notifier_block *self,
@@ -522,7 +521,7 @@ static struct notifier_block exynos4_mct_cpu_nb = {
 
 static void __init exynos4_timer_resources(struct device_node *np, void __iomem *base)
 {
-	int err;
+	int err, cpu;
 	struct mct_clock_event_device *mevt = this_cpu_ptr(&percpu_mct_tick);
 	struct clk *mct_clk, *tick_clk;
 
@@ -549,7 +548,27 @@ static void __init exynos4_timer_resources(struct device_node *np, void __iomem
 		WARN(err, "MCT: can't request IRQ %d (%d)\n",
 		     mct_irqs[MCT_L0_IRQ], err);
 	} else {
-		irq_set_affinity(mct_irqs[MCT_L0_IRQ], cpumask_of(0));
+		for_each_possible_cpu(cpu) {
+			int mct_irq = mct_irqs[MCT_L0_IRQ + cpu];
+			struct mct_clock_event_device *pcpu_mevt =
+				per_cpu_ptr(&percpu_mct_tick, cpu);
+
+			pcpu_mevt->evt.irq = -1;
+
+			if (request_irq(mct_irq,
+					exynos4_mct_tick_isr,
+					IRQF_TIMER | IRQF_NOBALANCING,
+					pcpu_mevt->name, pcpu_mevt)) {
+				pr_err("exynos-mct: cannot register IRQ (cpu%d)\n",
+									cpu);
+
+				continue;
+			}
+			pcpu_mevt->evt.irq = mct_irq;
+			irq_force_affinity(mct_irq, cpumask_of(cpu));
+
+			disable_irq(mct_irq);
+		}
 	}
 
 	err = register_cpu_notifier(&exynos4_mct_cpu_nb);
-- 
1.9.1

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