[PATCH v4 2/2] KVM: selftests: Wait for vcpu thread before signal setup

[Peter Xu <peterx@redhat.com>]

The main thread could start to send SIG_IPI at any time, even before signal
blocked on vcpu thread.  Reuse the sem_vcpu_stop to sync on that, so when
SIG_IPI is sent the signal will always land correctly as an -EINTR.

Without this patch, on very busy cores the dirty_log_test could fail directly
on receiving a SIG_USR1 without a handler (when vcpu runs far slower than main).

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 tools/testing/selftests/kvm/dirty_log_test.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/tools/testing/selftests/kvm/dirty_log_test.c b/tools/testing/selftests/kvm/dirty_log_test.c
index fc87e2f11d3d..d3050d1c2cd0 100644
--- a/tools/testing/selftests/kvm/dirty_log_test.c
+++ b/tools/testing/selftests/kvm/dirty_log_test.c
@@ -534,6 +534,12 @@ static void *vcpu_worker(void *data)
 	sigemptyset(sigset);
 	sigaddset(sigset, SIG_IPI);
 
+	/*
+	 * Tell the main thread that signals are setup already; let's borrow
+	 * sem_vcpu_stop even if it's not for it.
+	 */
+	sem_post(&sem_vcpu_stop);
+
 	guest_array = addr_gva2hva(vm, (vm_vaddr_t)random_array);
 
 	while (!READ_ONCE(host_quit)) {
@@ -785,6 +791,8 @@ static void run_test(enum vm_guest_mode mode, void *arg)
 
 	pthread_create(&vcpu_thread, NULL, vcpu_worker, vm);
 
+	sem_wait_until(&sem_vcpu_stop);
+
 	while (iteration < p->iterations) {
 		/* Give the vcpu thread some time to dirty some pages */
 		usleep(p->interval * 1000);
-- 
2.26.2