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

[Tianxiang Chen <nanmu@xiaomi.com>]

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@xiaomi.com>
---
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();
 }
 
 /**
-- 
2.34.1