Re: [PATCH v2] KVM: selftests: Fix the dirty_log_test semaphore imbalance

[Sean Christopherson <seanjc@google.com>]

+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@redhat.com>
> ---
> v1->v2:
>   - Fix the real logic bug, not just fresh the context.
> 
> v1: https://lore.kernel.org/all/20231116093536.22256-1-shahuang@redhat.com/
> ---
>  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.