Date: Thu, 1 Feb 2024 16:57:19 +0800
From: Shaoqin Huang <shahuang@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: kvm@...r.kernel.org, kvmarm@...ts.linux.dev,
 Paolo Bonzini <pbonzini@...hat.com>, Shuah Khan <shuah@...nel.org>,
 linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
 Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH v2] KVM: selftests: Fix the dirty_log_test semaphore
 imbalance

Hi Sean,

Thanks for your reviewing.

On 1/27/24 04:25, Sean Christopherson wrote:
> +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@...hat.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().
> 

Sure. I will replace them with 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.

If that's the case. I guess I should keep the current solution. Except 
you have any better solution, please let me know.

Thanks,
Shaoqin

> 

-- 
Shaoqin

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