Re: [PATCH v2] cpufreq: Fix hotplug-suspend race during reboot

[Zhongqiu Han]

On 4/8/2026 10:19 PM, Tianxiang Chen wrote:
> During system reboot, cpufreq_suspend() is called via the
> kernel_restart() -> device_shutdown() -> pm_notifier_call_chain()
> path. Unlike the normal system suspend path, the reboot path does not
> call freeze_processes(), so userspace processes and kernel threads
> remain active.
>
> This allows CPU hotplug operations to run concurrently with
> cpufreq_suspend(). The original code has no synchronization with CPU
> hotplug, leading to a race condition where governor_data can be freed
> by the hotplug path while cpufreq_suspend() is still accessing it,
> resulting in a null pointer dereference:
>
>    Unable to handle kernel NULL pointer dereference
>    Call Trace:
>     do_kernel_fault+0x28/0x3c
>     cpufreq_suspend+0xdc/0x160
>     device_shutdown+0x18/0x200
>     kernel_restart+0x40/0x80
>     arm64_sys_reboot+0x1b0/0x200
>
> Fix this by adding cpus_read_lock()/cpus_read_unlock() to
> cpufreq_suspend() to block CPU hotplug operations while suspend is in
> progress.
>
> Signed-off-by: Tianxiang Chen <nanmu@xxxxxxxxxx>
> ---
> v2:
> - Update changelog to explicitly mention reboot scenario
> - Add observed crash trace
> ---
>   drivers/cpufreq/cpufreq.c | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 1f794524a1d9..6f1d264c378b 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -1979,6 +1979,7 @@ void cpufreq_suspend(void)
>   	if (!cpufreq_driver)
>   		return;
>   
> +	cpus_read_lock();
>   	if (!has_target() && !cpufreq_driver->suspend)
>   		goto suspend;
>   
> @@ -1998,6 +1999,7 @@ void cpufreq_suspend(void)
>   
>   suspend:
>   	cpufreq_suspended = true;
> +	cpus_read_unlock();
>   }
>   
>   /**

Hi Tianxiang,

May I know did you test this with lockdep enabled? Specifically, does
the new cpus_read_lock() → policy->rwsem ordering in cpufreq_suspend()
trigger any lockdep warnings? Thanks



--
Thx and BRs,
Zhongqiu Han