Re: [PATCH v11 33/46] KVM: selftests: Hyper-V PV IPI selftest

Vitaly Kuznetsov
2022-10-21T13:43:42Z

Vitaly Kuznetsov <vkuznets@redhat.com> writes:

> Sean Christopherson <seanjc@google.com> writes:
>
>> On Tue, Oct 04, 2022, Vitaly Kuznetsov wrote:
>
> ...
>
>>> +
>>> + r = pthread_cancel(thread);
>>> + TEST_ASSERT(r == 0,
>>
>> !r is generally preferred over "r == 0"
>>
>>> + "pthread_cancel on vcpu_id=%d failed with errno=%d",
>>> + vcpu->id, r);
>>
>> Do you happen to know if errno is preserved? I.e. if TEST_ASSERT()'s print of
>> errno will capture the right errno? If so, this and the pthread_join() assert
>> can be:
>>
>> TEST_ASSERT(!r, pthread_cancel() failed on vcpu_id=%d, vcpu->id);
>>
>
> The example from 'man 3 pthread_cancel' makes me think errno is not
> set. 'man 3 errno' confirms that:
>
> "
> Note that the POSIX threads APIs do not set errno on error.
> Instead, on failure they return an error number as the function result.
> These error numbers have the same meanings as the error numbers returned
> in errno by other APIs.
> "
>
> but nothing stops us from doing something like
>
> #include <errno.h>
> ...
>
> errno = pthread_cancel(thread);
> TEST_ASSERT(!errno, pthread_cancel() failed on vcpu_id=%d, vcpu->id);
>
> I believe.

... only the fact that this won't be thread safe :-( i.e. if we also try
setting 'errno' from vcpu_thread() (where the pattern for
pthread_setcanceltype() is exactly the same), we will likely be
reporting the wrong errno. I think it's better to keep reporting 'r' for
now (and maybe think about pthread* wrappers later).

--
Vitaly