On Tue, Feb 14, 2012 at 11:53 AM, Alex Barcelo <abarc...@ac.upc.edu> wrote:
> On Tue, Feb 14, 2012 at 10:24, Stefan Hajnoczi <stefa...@gmail.com> wrote:
>> On Mon, Feb 13, 2012 at 03:42:28PM +0100, Alex Barcelo wrote:
>>> + /*
>>> + * Preserve the SIGUSR1 signal state, block SIGUSR1,
>>> + * and establish our signal handler. The signal will
>>> + * later transfer control onto the signal stack.
>>> + */
>>> + sigemptyset(&sigs);
>>> + sigaddset(&sigs, SIGUSR1);
>>> + sigprocmask(SIG_BLOCK, &sigs, &osigs);
>>> + sa.sa_handler = coroutine_trampoline;
>>> + sigfillset(&sa.sa_mask);
>>> + sa.sa_flags = SA_ONSTACK;
>>> + if (sigaction(SIGUSR1, &sa, &osa) != 0) {
>>> + abort();
>>> + }
>>> +
>>> + /*
>>> + * Set the new stack.
>>> + */
>>> + ss.ss_sp = co->stack;
>>> + ss.ss_size = stack_size;
>>> + ss.ss_flags = 0;
>>> + if (sigaltstack(&ss, &oss) < 0) {
>>> + abort();
>>> + }
>>> +
>>> + /*
>>> + * Now transfer control onto the signal stack and set it up.
>>> + * It will return immediately via "return" after the setjmp()
>>> + * was performed. Be careful here with race conditions. The
>>> + * signal can be delivered the first time sigsuspend() is
>>> + * called.
>>> + */
>>> + tr_called = 0;
>>> + kill(getpid(), SIGUSR1);
>>> + sigfillset(&sigs);
>>> + sigdelset(&sigs, SIGUSR1);
>>> + while (!tr_called) {
>>> + sigsuspend(&sigs);
>>> + }
>>> +
>>> + /*
>>> + * Inform the system that we are back off the signal stack by
>>> + * removing the alternative signal stack. Be careful here: It
>>> + * first has to be disabled, before it can be removed.
>>> + */
>>> + sigaltstack(NULL, &ss);
>>
>> What happens when a vcpu thread creates a coroutine while another QEMU
>> thread raises SIG_IPI? The SIG_IPI will be handled on the alternate
>> signal stack
>
> mmm no, it won't. The sigaction is set for the SIGUSR1 only (yes I
> have to change it to sigusr2, the V2 will have this changed). And only
> this signal will be handled on an alternate stack (the sa.sa_flags is
> the responsible).
>
> I'm not really sure about that, did I miss something?
What I meant is that there are other signals handlers installed and
the signals will be unblocked between the time when sigsuspend() has
returned and before sigaltstack(NULL, &ss) is executed. This seems
like a race condition to me.
Stefan
