[PATCH] x86/aperfmperf: Fix deadlock on cpu_hotplug_lock

[Jonathan Cameron]

The broken patch results in a call to init_freq_invariance_cppc() in a CPU
hotplug handler in both the path for initially present CPUs and those
hotplugged later.  That function includes a one time call to
amd_set_max_freq_ratio() which in turn calls freq_invariance_enable() that
has a static_branch_enable() which takes the cpu_hotlug_lock which is
already held.

Avoid the deadlock by using static_branch_enable_cpuslocked() as the lock
will always be already held.  The equivalent path on Intel does not
already hold this lock, so take it around the call to
freq_invariance_enable(), which results in it being held over the call to
register_syscall_ops, which looks to be safe to do.

Fixes: c1385c1f0ba3 ("ACPI: processor: Simplify initial onlining to use same path for cold and hotplug")
Reported-by: Mikhail Gavrilov <mikhail.v.gavrilov@xxxxxxxxx>
Closes: https://lore.kernel.org/all/CABXGCsPvqBfL5hQDOARwfqasLRJ_eNPBbCngZ257HOe=xbWDkA@xxxxxxxxxxxxxx/
Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Tested-by: Mikhail Gavrilov <mikhail.v.gavrilov@xxxxxxxxx>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
---
 arch/x86/kernel/cpu/aperfmperf.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

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)
-- 
2.43.0