Re: [PATCH] clocksource/arm_arch_timer: fix a lockdep warning

[Waiman Long <longman@redhat.com>]

On 12/03/2018 02:33 PM, 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"). Therefore, it will check
> if it is really in the CPU hotplug path or not, and work around this
> problem by using cpus_read_trylock(). The chance of not getting the read
> lock is very small. If that happens, it will report a lockdep warning at
> most.
>
> Signed-off-by: Qian Cai <cai@gmx.us>
> ---
>  drivers/clocksource/arm_arch_timer.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
> index 9a7d4dc..5c9acbd 100644
> --- a/drivers/clocksource/arm_arch_timer.c
> +++ b/drivers/clocksource/arm_arch_timer.c
> @@ -497,11 +497,20 @@ void arch_timer_enable_workaround(const struct arch_timer_erratum_workaround *wa
>  			per_cpu(timer_unstable_counter_workaround, i) = wa;
>  	}
>  
> +#ifdef CONFIG_HOTPLUG_CPU

If HOTPLUG_CPU isn't defined, all the cpus_lock() and related functions
are just no-op. You don't need to use conditional compilation directive
here.

> +	i = 0;
> +
>  	/*
>  	 * Use the locked version, as we're called from the CPU
>  	 * hotplug framework. Otherwise, we end-up in deadlock-land.
>  	 */

I think the main problem is the above comment may not be true anymore or
is only occasionally true. We need to audit the code to find the root cause.

> +	i = cpus_read_trylock();
>  	static_branch_enable_cpuslocked(&arch_timer_read_ool_enabled);
> +	if (i)
> +		cpus_read_unlock();

This is not the right way of fixing the lockdep splash.

> +#else
> +	static_branch_enable(&arch_timer_read_ool_enabled);
> +#endif
>  
>  	/*
>  	 * Don't use the vdso fastpath if errata require using the

Cheers,
Longman