Re: [PATCH v3 2/2] KVM: selftests: Wait for vcpu thread before signal setup

[Paolo Bonzini <pbonzini@redhat.com>]

On 17/04/21 16:36, Peter Xu wrote:
> 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>

This should be a better way to do the same:

----------- 8< ------------
From: Paolo Bonzini <pbonzini@redhat.com>
Subject: [PATCH] KVM: selftests: Wait for vcpu thread before signal setup

The main thread could start to send SIG_IPI at any time, even before signal
blocked on vcpu thread.  Therefore, start the vcpu thread with the signal
blocked.

Without this patch, on very busy cores the dirty_log_test could fail directly
on receiving a SIGUSR1 without a handler (when vcpu runs far slower than main).

Reported-by: Peter Xu <peterx@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/tools/testing/selftests/kvm/dirty_log_test.c b/tools/testing/selftests/kvm/dirty_log_test.c
index ffa4e2791926..048973d50a45 100644
--- a/tools/testing/selftests/kvm/dirty_log_test.c
+++ b/tools/testing/selftests/kvm/dirty_log_test.c
@@ -527,9 +527,8 @@ static void *vcpu_worker(void *data)
  	 */
  	sigmask->len = 8;
  	pthread_sigmask(0, NULL, sigset);
+	sigdelset(sigset, SIG_IPI);
  	vcpu_ioctl(vm, VCPU_ID, KVM_SET_SIGNAL_MASK, sigmask);
-	sigaddset(sigset, SIG_IPI);
-	pthread_sigmask(SIG_BLOCK, sigset, NULL);
  
  	sigemptyset(sigset);
  	sigaddset(sigset, SIG_IPI);
@@ -858,6 +857,7 @@ int main(int argc, char *argv[])
  		.interval = TEST_HOST_LOOP_INTERVAL,
  	};
  	int opt, i;
+	sigset_t sigset;
  
  	sem_init(&sem_vcpu_stop, 0, 0);
  	sem_init(&sem_vcpu_cont, 0, 0);
@@ -916,6 +916,11 @@ int main(int argc, char *argv[])
  
  	srandom(time(0));
  
+	/* Ensure that vCPU threads start with SIG_IPI blocked.  */
+	sigemptyset(&sigset);
+	sigaddset(&sigset, SIG_IPI);
+	pthread_sigmask(SIG_BLOCK, sigset, NULL);
+
  	if (host_log_mode_option == LOG_MODE_ALL) {
  		/* Run each log mode */
  		for (i = 0; i < LOG_MODE_NUM; i++) {


> ---
>   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 510884f0eab8..25230e799bc4 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);
>