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

[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@redhat.com>
---
 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

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

[Oliver Upton]

Hi Shaoqin,

On Thu, Nov 16, 2023 at 04:35:36AM -0500, 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.
> 
> 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.

In your changelog you describe what sounds like a semaphore imbalance at
the time of test completion, yet your proposed fix is to just clobber
the error and start fresh.

Why not nip it at the bud and fix the logic bug instead?

-- 
Thanks,
Oliver

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

[Shaoqin Huang]

Hi Oliver,

On 11/17/23 08:18, Oliver Upton wrote:
> Hi Shaoqin,
> 
> On Thu, Nov 16, 2023 at 04:35:36AM -0500, 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.
>>
>> 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.
> 
> In your changelog you describe what sounds like a semaphore imbalance at
> the time of test completion, yet your proposed fix is to just clobber
> the error and start fresh.
> 

Yes. It's a semaphore imbalance problem.

> Why not nip it at the bud and fix the logic bug instead?

I have another patch which fix the logic bug, I will send it out later.

> 

-- 
Shaoqin