[bug report] KVM: x86: Unify TSC logic (sleeping in atomic?)

[Dan Carpenter <dan.carpenter@xxxxxxxxxx>]

I don't know why I'm seeing this static checker warning only now.  All
this code looks to be about 15 years old...

Commit e48672fa25e8 ("KVM: x86: Unify TSC logic") from Aug 19, 2010
(linux-next), leads to the following Smatch static checker warning:

	arch/x86/kernel/tsc.c:1214 mark_tsc_unstable()
	warn: sleeping in atomic context

The code path is:

vcpu_load() <- disables preempt
-> kvm_arch_vcpu_load()
   -> mark_tsc_unstable() <- sleeps

virt/kvm/kvm_main.c
   166  void vcpu_load(struct kvm_vcpu *vcpu)
   167  {
   168          int cpu = get_cpu();
                          ^^^^^^^^^^
This get_cpu() disables preemption.

   169  
   170          __this_cpu_write(kvm_running_vcpu, vcpu);
   171          preempt_notifier_register(&vcpu->preempt_notifier);
   172          kvm_arch_vcpu_load(vcpu, cpu);
   173          put_cpu();
   174  }

arch/x86/kvm/x86.c
  4979          if (unlikely(vcpu->cpu != cpu) || kvm_check_tsc_unstable()) {
  4980                  s64 tsc_delta = !vcpu->arch.last_host_tsc ? 0 :
  4981                                  rdtsc() - vcpu->arch.last_host_tsc;
  4982                  if (tsc_delta < 0)
  4983                          mark_tsc_unstable("KVM discovered backwards TSC");
                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
It seems pretty unlikely that we'll get a backwards tsc.  I remember
dealing with some of these seeing this 20 years ago.

  4984  

arch/x86/kernel/tsc.c
    1206 void mark_tsc_unstable(char *reason)
    1207 {
    1208         if (tsc_unstable)
    1209                 return;
    1210 
    1211         tsc_unstable = 1;
    1212         if (using_native_sched_clock())
    1213                 clear_sched_clock_stable();
--> 1214         disable_sched_clock_irqtime();
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
I'm on an allmodconfig on x86_64 so this expands to the
function below...

    1215         pr_info("Marking TSC unstable due to %s\n", reason);
    1216 
    1217         clocksource_mark_unstable(&clocksource_tsc_early);
    1218         clocksource_mark_unstable(&clocksource_tsc);
    1219 }

kernel/jump_label.c
   245  void static_key_disable(struct static_key *key)
   246  {
   247          cpus_read_lock();
                ^^^^^^^^^^^^^^^^
This lock has a might_sleep() in it which triggers the static checker
warning.

   248          static_key_disable_cpuslocked(key);
   249          cpus_read_unlock();
   250  }

regards,
dan carpenter