Re: [PATCH v2 2/2] KVM: selftests: Randomize pCPU in steal time test

[Sean Christopherson <seanjc@google.com>]

On Tue, Jul 07, 2026, sashiko-bot@kernel.org wrote:
> > diff --git a/tools/testing/selftests/kvm/steal_time.c b/tools/testing/selftests/kvm/steal_time.c
> > --- a/tools/testing/selftests/kvm/steal_time.c
> > +++ b/tools/testing/selftests/kvm/steal_time.c
> [ ... ]
> > @@ -514,14 +514,12 @@ int main(int ac, char **av)
> >  	long stolen_time;
> >  	long run_delay;
> >  	bool verbose;
> > -	int i;
> > +	int i, cpu;
> >  
> >  	verbose = ac > 1 && (!strncmp(av[1], "-v", 3) || !strncmp(av[1], "--verbose", 10));
> >  
> >  	/* Set CPU affinity so we can force preemption of the VCPU */
> > -	CPU_ZERO(&cpuset);
> > -	CPU_SET(0, &cpuset);
> > -	pthread_setaffinity_np(pthread_self(), sizeof(cpu_set_t), &cpuset);
> > +	cpu = pin_self_to_any_cpu();
> >  
> >  	/* Create a VM and an identity mapped memslot for the steal time structure */
> >  	vm = vm_create_with_vcpus(NR_VCPUS, guest_code, vcpus);
> [ ... ]
> > @@ -557,7 +555,7 @@ int main(int ac, char **av)
> >  		run_delay = get_run_delay();
> >  		pthread_create(&thread, NULL, do_steal_time, NULL);
> >  		pthread_getaffinity_np(thread, sizeof(cpuset), &cpuset);
> > -		TEST_ASSERT(CPU_COUNT(&cpuset) == 1 && CPU_ISSET(0, &cpuset),
> > +		TEST_ASSERT(CPU_COUNT(&cpuset) == 1 && CPU_ISSET(cpu, &cpuset),
> 
> [Severity: Medium]
> Is there a race condition here if the worker thread exits before the parent
> reads its affinity?
> 
> When pthread_create() spawns the worker thread, the child runs for a short
> duration and exits. If the parent thread is preempted right after creation,
> the child might finish its work and exit before pthread_getaffinity_np()
> executes in the parent.
> 
> If the kernel task is already gone, pthread_getaffinity_np() will fail with

No, because the kernel task is kept alive until pthread_join().  As per the
pthread_create() manpage[*]:

  Only when a terminated joinable thread has been joined are the last of its
  resources released back to the system.

And by empirical testing, that includes the kernel task.

[*] https://man7.org/linux/man-pages/man3/pthread_create.3.html

> ESRCH. Because the explicit initialization of cpuset was removed earlier in
> main(), cpuset remains completely uninitialized.
> 
> Could this cause CPU_COUNT() and CPU_ISSET() to evaluate garbage stack memory
> and trigger sporadic TEST_ASSERT failures? Should the return value of
> pthread_getaffinity_np() be checked, or should cpuset be initialized before
> the call?

The return value should be checked, but I'd rather handle that in a separate
cleanup by adding a wrapper in kvm_syscalls.h.