+sched folks
On Fri, Jan 17, 2025, Dan Carpenter wrote:
> 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:
That's not the problematic commit. This popped because commit 8722903cbb8f
("sched: Define sched_clock_irqtime as static key") in the tip tree turned
sched_clock_irqtime into a static key (it was a simple "int").
https://lore.kernel.org/all/20250103022409.2544-2-laoar.shao@xxxxxxxxx
> 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.
TSC will go "backwards" if the host is suspended, in which case host TSC gets
reset to 0.
> 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
