linux-kernel - Re: [RESEND PATCH v2] clocksource/arm_arch

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Date:   Mon, 10 Dec 2018 14:48:03 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Qian Cai <cai@....pw>, akpm@...ux-foundation.org
Cc:     mark.rutland@....com, daniel.lezcano@...aro.org,
        tglx@...utronix.de, peterz@...radead.org, mingo@...nel.org,
        longman@...hat.com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH v2] clocksource/arm_arch_timer: fix a lockdep
 warning

On 10/12/2018 13:52, Qian Cai wrote:
> Booting this Huawei TaiShan 2280 arm64 server generated this lockdep
> warning.
> 
> [    0.000000]  lockdep_assert_cpus_held+0x50/0x60
> [    0.000000]  static_key_enable_cpuslocked+0x30/0xe8
> [    0.000000]  arch_timer_check_ool_workaround+0x128/0x2d0
> [    0.000000]  arch_timer_acpi_init+0x274/0x6ac
> [    0.000000]  acpi_table_parse+0x1ac/0x218
> [    0.000000]  __acpi_probe_device_table+0x164/0x1ec
> [    0.000000]  timer_probe+0x1bc/0x254
> [    0.000000]  time_init+0x44/0x98
> [    0.000000]  start_kernel+0x4ec/0x7d4
> 
> This is due to the commit cb538267ea1e ("jump_label/lockdep: Assert we hold
> the hotplug lock for _cpuslocked() operations").
> 
> Since it is applying a global workaround to all CPUs here, it did not hold
> any CPU locks in this path.
> 
> arch_timer_acpi_init
>   arch_timer_check_ool_workaround(ate_match_acpi_oem_info, table)
>     arch_timer_enable_workaround(wa, local = false)
>       for_each_possible_cpu()
> 	per_cpu()
> 
> There is also another path did not have any CPU lock.
> 
> time_init
>   clocksource_probe
>     arch_timer_of_init
>       arch_timer_check_ool_workaround(ate_match_dt, np)
>         arch_timer_enable_workaround(wa, local = false)
> 
> When hot-adding a CPU, it will go with a slightly different route.
> 
> arch_timer_starting_cpu
>   __arch_timer_setup
>     arch_timer_check_ool_workaround(ate_match_local_cap_id, NULL)
>       arch_timer_enable_workaround(wa, local = true)
>         __this_cpu_write()
> 
> Hence, deal with them differently.
> 
> Fixes: 450f9689f294 (clocksource/arm_arch_timer: Use
> static_branch_enable_cpuslocked())
> Signed-off-by: Qian Cai <cai@....pw>
> ---
> 
> v2: fix the root cause instead of a workaround.
> 
>  drivers/clocksource/arm_arch_timer.c | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
> index 9a7d4dc00b6e..81dca7d31d13 100644
> --- a/drivers/clocksource/arm_arch_timer.c
> +++ b/drivers/clocksource/arm_arch_timer.c
> @@ -492,17 +492,20 @@ void arch_timer_enable_workaround(const struct arch_timer_erratum_workaround *wa
>  
>  	if (local) {
>  		__this_cpu_write(timer_unstable_counter_workaround, wa);
> +
> +		/*
> +		 * Use the locked version, as we're called from the CPU
> +		 * hotplug framework. Otherwise, we end-up in
> +		 * deadlock-land.
> +		 */
> +		static_branch_enable_cpuslocked(&arch_timer_read_ool_enabled);

I have the ugly feeling that it breaks the (equally ugly) big-little
stuff where the boot CPU is affected. In this context, you'd end-up
without the lock being taken and you'll get the same splat.

We could start testing the CPU number, but Peter's approach seem much
more palatable to me.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...