linux-kernel - Re: [PATCH v2 2/5] cpuidle: psci: Mark as PREEMPT

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Message-ID: <Y7/oO8s26SKU8eOz@linutronix.de>
Date:   Thu, 12 Jan 2023 12:00:11 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kevin Hilman <khilman@...nel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Sudeep Holla <sudeep.holla@....com>, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Adrien Thierry <athierry@...hat.com>,
        Brian Masney <bmasney@...hat.com>,
        linux-rt-users@...r.kernel.org
Subject: Re: [PATCH v2 2/5] cpuidle: psci: Mark as PREEMPT_RT safe

On 2022-12-19 16:15:00 [+0100], Krzysztof Kozlowski wrote:
> The PSCI cpuidle power domain in power_off callback uses
> __this_cpu_write() so it is PREEMPT_RT safe.  This allows to use it in

Why does __this_cpu_write() matter here?

> Realtime kernels and solves errors like:
> 
>   BUG: scheduling while atomic: swapper/2/0/0x00000002
>   Hardware name: Qualcomm Technologies, Inc. Robotics RB5 (DT)
>   Call trace:
>    dump_backtrace.part.0+0xe0/0xf0
>    show_stack+0x18/0x40
>    dump_stack_lvl+0x68/0x84
>    dump_stack+0x18/0x34
>    __schedule_bug+0x60/0x80
>    __schedule+0x628/0x800
>    schedule_rtlock+0x28/0x5c
>    rtlock_slowlock_locked+0x360/0xd30
>    rt_spin_lock+0x88/0xb0
>    genpd_lock_nested_spin+0x1c/0x30
>    genpd_power_off.part.0.isra.0+0x20c/0x2a0
>    genpd_runtime_suspend+0x150/0x2bc
>    __rpm_callback+0x48/0x170
>    rpm_callback+0x6c/0x7c
>    rpm_suspend+0x108/0x660
>    __pm_runtime_suspend+0x4c/0x8c
>    __psci_enter_domain_idle_state.constprop.0+0x54/0xe0
>    psci_enter_domain_idle_state+0x18/0x2c
>    cpuidle_enter_state+0x8c/0x4e0
>    cpuidle_enter+0x38/0x50
>    do_idle+0x248/0x2f0
>    cpu_startup_entry+0x24/0x30
>    secondary_start_kernel+0x130/0x154
>    __secondary_switched+0xb0/0xb4

This is to a sleeping lock (spinlock_t) in an IRQ-off region which
starts in do_idle(). You could describe the problem and to solution you
aim for instead pasting a backtrace into the commit description and
adding a flow in the code.

I don't see how your commit description matches your change in code. One
might think "Oh. If I see this pattern, I need an irqsafe lock to fix
it".

Sebastian