+Peter On Fri, Nov 17, 2023, Shaoqin Huang wrote: > 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. > > To fix this problem, we can wait a while before set the host_quit to > true, which gives the vcpu time to enter the guest state, so it will > exit again. Then we can wait the vcpu to exit, and let it continue > again, then the vcpu will see the host_quit. Thus the sem_vcpu_cont and > sem_vcpu_stop will be both zero when test finished. > > Signed-off-by: Shaoqin Huang <shahuang@xxxxxxxxxx> > --- > v1->v2: > - Fix the real logic bug, not just fresh the context. > > v1: https://lore.kernel.org/all/20231116093536.22256-1-shahuang@xxxxxxxxxx/ > --- > tools/testing/selftests/kvm/dirty_log_test.c | 16 +++++++++++++++- > 1 file changed, 15 insertions(+), 1 deletion(-) > > diff --git a/tools/testing/selftests/kvm/dirty_log_test.c b/tools/testing/selftests/kvm/dirty_log_test.c > index 936f3a8d1b83..a6e0ff46a07c 100644 > --- a/tools/testing/selftests/kvm/dirty_log_test.c > +++ b/tools/testing/selftests/kvm/dirty_log_test.c > @@ -417,7 +417,8 @@ static void dirty_ring_after_vcpu_run(struct kvm_vcpu *vcpu, int ret, int err) > > static void dirty_ring_before_vcpu_join(void) > { > - /* Kick another round of vcpu just to make sure it will quit */ > + /* Wait vcpu exit, and let it continue to see the host_quit. */ > + dirty_ring_wait_vcpu(); > sem_post(&sem_vcpu_cont); > } > > @@ -719,6 +720,7 @@ static void run_test(enum vm_guest_mode mode, void *arg) > struct kvm_vm *vm; > unsigned long *bmap; > uint32_t ring_buf_idx = 0; > + int sem_val; > > if (!log_mode_supported()) { > print_skip("Log mode '%s' not supported", > @@ -726,6 +728,11 @@ static void run_test(enum vm_guest_mode mode, void *arg) > return; > } > > + sem_getvalue(&sem_vcpu_stop, &sem_val); > + assert(sem_val == 0); > + sem_getvalue(&sem_vcpu_cont, &sem_val); > + assert(sem_val == 0); Never use bare assert() in selftests, they'll either get compiled out or IIRC, will cause the test to silently exit if they fail. Either do nothing, or use TEST_ASSERT(). > + > /* > * We reserve page table for 2 times of extra dirty mem which > * will definitely cover the original (1G+) test range. Here > @@ -825,6 +832,13 @@ static void run_test(enum vm_guest_mode mode, void *arg) > sync_global_to_guest(vm, iteration); > } > > + /* > + * > + * Before we set the host_quit, let the vcpu has time to run, to make > + * sure we consume the sem_vcpu_stop and the vcpu consume the > + * sem_vcpu_cont, to keep the semaphore balance. > + */ > + usleep(p->interval * 1000); Please no. "Wait for a while" is never a complete solution for fixing races. In rare cases, adding a delay might be the only sane workaround, but I doubt that's the case here.
