[PATCH v1] KVM: selftests: Initalize sem_vcpu_[cont|stop] before each test in dirty_log_test

[Shaoqin Huang]

When execute the dirty_log_test on some aarch64 machine, it sometimes
trigger the ASSERT:

==== Test Assertion Failure ====
  dirty_log_test.c:384: dirty_ring_vcpu_ring_full
  pid=14854 tid=14854 errno=22 - Invalid argument
     1  0x00000000004033eb: dirty_ring_collect_dirty_pages at dirty_log_test.c:384
     2  0x0000000000402d27: log_mode_collect_dirty_pages at dirty_log_test.c:505
     3   (inlined by) run_test at dirty_log_test.c:802
     4  0x0000000000403dc7: for_each_guest_mode at guest_modes.c:100
     5  0x0000000000401dff: main at dirty_log_test.c:941 (discriminator 3)
     6  0x0000ffff9be173c7: ?? ??:0
     7  0x0000ffff9be1749f: ?? ??:0
     8  0x000000000040206f: _start at ??:?
  Didn't continue vcpu even without ring full

The dirty_log_test fails when execute the dirty-ring test, this is
because the sem_vcpu_cont and the sem_vcpu_stop is non-zero value when
execute the dirty_ring_collect_dirty_pages() function. When those two
sem_t variables are non-zero, the dirty_ring_wait_vcpu() at the
beginning of the dirty_ring_collect_dirty_pages() will not wait for the
vcpu to stop, but continue to execute the following code. In this case,
before vcpu stop, if the dirty_ring_vcpu_ring_full is true, and the
dirty_ring_collect_dirty_pages() has passed the check for the
dirty_ring_vcpu_ring_full but hasn't execute the check for the
continued_vcpu, the vcpu stop, and set the dirty_ring_vcpu_ring_full to
false. Then dirty_ring_collect_dirty_pages() will trigger the ASSERT.

Why sem_vcpu_cont and sem_vcpu_stop can be non-zero value? It's because
the dirty_ring_before_vcpu_join() execute the sem_post(&sem_vcpu_cont)
at the end of each dirty-ring test. It can cause two cases:

1. sem_vcpu_cont be non-zero. When we set the host_quit to be true,
   the vcpu_worker directly see the host_quit to be true, it quit. So
   the log_mode_before_vcpu_join() function will set the sem_vcpu_cont
   to 1, since the vcpu_worker has quit, it won't consume it.
2. sem_vcpu_stop be non-zero. When we set the host_quit to be true,
   the vcpu_worker has entered the guest state, the next time it exit
   from guest state, it will set the sem_vcpu_stop to 1, and then see
   the host_quit, no one will consume the sem_vcpu_stop.

When execute more and more dirty-ring tests, the sem_vcpu_cont and
sem_vcpu_stop can be larger and larger, which makes many code paths
don't wait for the sem_t. Thus finally cause the problem.

Fix this problem is easy, simply initialize the sem_t before every test.
Thus whatever the state previous test left, it won't interfere the next
test.

Signed-off-by: Shaoqin Huang <shahuang@xxxxxxxxxx>
---
 tools/testing/selftests/kvm/dirty_log_test.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/kvm/dirty_log_test.c b/tools/testing/selftests/kvm/dirty_log_test.c
index 936f3a8d1b83..23b179534c0b 100644
--- a/tools/testing/selftests/kvm/dirty_log_test.c
+++ b/tools/testing/selftests/kvm/dirty_log_test.c
@@ -726,6 +726,9 @@ static void run_test(enum vm_guest_mode mode, void *arg)
 		return;
 	}
 
+	sem_init(&sem_vcpu_stop, 0, 0);
+	sem_init(&sem_vcpu_cont, 0, 0);
+
 	/*
 	 * We reserve page table for 2 times of extra dirty mem which
 	 * will definitely cover the original (1G+) test range.  Here
@@ -871,9 +874,6 @@ int main(int argc, char *argv[])
 	int opt, i;
 	sigset_t sigset;
 
-	sem_init(&sem_vcpu_stop, 0, 0);
-	sem_init(&sem_vcpu_cont, 0, 0);
-
 	guest_modes_append_default();
 
 	while ((opt = getopt(argc, argv, "c:hi:I:p:m:M:")) != -1) {
-- 
2.40.1