Re: 6.11/regression/bisected - The commit c1385c1f0ba3 caused a new possible recursive locking detected warning at computer boot.

[Thomas Gleixner]

On Thu, Jul 25 2024 at 18:13, Jonathan Cameron wrote:
> On Tue, 23 Jul 2024 18:20:06 +0100
> Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> wrote:
>
>> > This is an interesting corner and perhaps reflects a flawed
>> > assumption we were making that for this path anything that can happen for an
>> > initially present CPU can also happen for a hotplugged one. On the hotplugged
>> > path the lock was always held and hence the static_key_enable() would
>> > have failed.

No. The original code invoked this without cpus read locked via:

acpi_processor_driver.probe()
   __acpi_processor_start()
       ....

and the cpu hotplug callback finds it already set up, so it won't reach
the static_key_enable() anymore.

> One bit I need to check out tomorrow is to make sure this doesn't race with the
> workfn that is used to tear down the same static key on error.

There is a simpler solution for that. See the uncompiled below.

Thanks,

        tglx
---
diff --git a/arch/x86/kernel/cpu/aperfmperf.c b/arch/x86/kernel/cpu/aperfmperf.c
index b3fa61d45352..0b69bfbf345d 100644
--- a/arch/x86/kernel/cpu/aperfmperf.c
+++ b/arch/x86/kernel/cpu/aperfmperf.c
@@ -306,7 +306,7 @@ static void freq_invariance_enable(void)
 		WARN_ON_ONCE(1);
 		return;
 	}
-	static_branch_enable(&arch_scale_freq_key);
+	static_branch_enable_cpuslocked(&arch_scale_freq_key);
 	register_freq_invariance_syscore_ops();
 	pr_info("Estimated ratio of average max frequency by base frequency (times 1024): %llu\n", arch_max_freq_ratio);
 }
@@ -323,8 +323,10 @@ static void __init bp_init_freq_invariance(void)
 	if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL)
 		return;
 
-	if (intel_set_max_freq_ratio())
+	if (intel_set_max_freq_ratio()) {
+		guard(cpus_read_lock)();
 		freq_invariance_enable();
+	}
 }
 
 static void disable_freq_invariance_workfn(struct work_struct *work)