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

Damian Eppel
2015-05-25T15:24:46Z

On Mon, 2015-05-11 at 13:18 +0200, Daniel Lezcano wrote:
> On 03/12/2015 10:11 AM, Damian Eppel wrote:
> > 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.
>
> The code sounds very complex for what it is supposed to do.
>
> Perhaps I am missing something but you have more or less the same
> functionality than the smp_twd timers and these ones don't look so complex.
>
> Could you please look at the smp_twd.c implementation ?

Hi Daniel,

exynos_mct.c driver looks more complex as it supports two types of timer
interrupts - private and shared peripheral interrupts (for exynos4412
and exynos4210 accordingly). In smp_twd.c driver I can see only PPI type
of irqs supported. SPI and PPI irqs differs slightly in setup - thus two
different code paths appears in the driver in initialization and
handling of CPU notifications. The fix is addressing issue that appears
only for hardware using SPI irqs so it is hard to compare it to
smp_twd.c.
BTW, If we remove support for SPI irqs in exynos_mct.c it would look
almost the same as smp_twd.c.

Best Regards,
Damian
>
>
> > Fixes: 7114cd749a12 ("clocksource: exynos_mct: use (request/free)_irq calls for local timer registration")
> > Signed-off-by: Damian Eppel <d.eppel@samsung.com>
> > Cc: <stable@vger.kernel.org>
> > Reported-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
> > Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
> > Tested-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
> > (Tested on Arndale Octa Board)
> > Tested-by: Marcin Jabrzyk <m.jabrzyk@samsung.com>
> > (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);
> >
>
>