Re: [Qemu-devel] Build broken

[Kevin Wolf <kwolf@redhat.com>]

Am 05.08.2011 10:48, schrieb Stefan Hajnoczi:
> On Fri, Aug 5, 2011 at 8:29 AM, Kevin Wolf <kwolf@redhat.com> wrote:
>> Am 05.08.2011 08:22, schrieb malc:
>>>
>>> /home/malc/x/rcs/git/qemuorg/coroutine-ucontext.c: In function 'coroutine_new':
>>> /home/malc/x/rcs/git/qemuorg/coroutine-ucontext.c:160:16: error: 'arg.i[1]' may be used uninitialized in this function
>>> /home/malc/x/rcs/git/qemuorg/coroutine-ucontext.c:136:18: note: 'arg.i[1]' was declared here
>>>
>>> diff --git a/coroutine-ucontext.c b/coroutine-ucontext.c
>>> index 41c2379..42dc3e2 100644
>>> --- a/coroutine-ucontext.c
>>> +++ b/coroutine-ucontext.c
>>> @@ -133,7 +133,7 @@ static Coroutine *coroutine_new(void)
>>>      CoroutineUContext *co;
>>>      ucontext_t old_uc, uc;
>>>      jmp_buf old_env;
>>> -    union cc_arg arg;
>>> +    union cc_arg arg = {0};
>>>
>>>      /* The ucontext functions preserve signal masks which incurs a system call
>>>       * overhead.  setjmp()/longjmp() does not preserve signal masks but only
>>>
>>> I guess gcc should yell not only here on ppc32 but on any machine where
>>> pointer size is less than the size of two ints.
>>
>> Stefan, why does this code even exist again? I think at some point I had
>> it changed to just use a static variable in order to avoid doing this
>> kind of tricks with unions.
> 
> virtfs are using coroutines in multiple threads at the same time.
> Introducing a global variable wouldn't be thread-safe.
> 
> The real problem is that makecontext(3) has a bad function signature.
> There's no nice fix - whatever we do will be ugly.
> 
> Using a union is the way it should be done in C.  The code doesn't
> look pretty but it doesn't introduce global state.

But it makes assumptions about the pointer size, which isn't a nice
thing. TLS isn't an option for compatibility with some OSes/architectures?

Kevin