Re: [PATCH 7/9] Enqueue signal even when resuming threads

Yao Qi
2016-07-01T16:45:37Z

On Fri, Jul 1, 2016 at 4:06 PM, Pedro Alves <palves@redhat.com> wrote:
> On 06/30/2016 03:09 PM, Yao Qi wrote:
>> Nowadays, we only enqueue signal when we leave thread pending in
>> linux_resume_one_thread. If lwp->resume->sig isn't zero (GDB wants
>> to resume with signal), we pass lwp->resume->sig to
>> linux_resume_one_lwp.
>>
>> In order to reduce the difference between resuming thread with signal
>> and proceeding thread with signal, when we resume thread, we can
>> enqueue signal too, and proceed thread. The signal will be consumed in
>> linux_resume_one_lwp_throw from lwp->pending_signals.
>
> This makes one subtle change. If the thread already
> had a pending signal, and we're getting a resume request with
> a signal that is a different signal from the one already queued,
> then before the patch, we'd tell the kernel to deliver the new
> signal first, and then only deliver the pending signal the next time
> the thread is resumed. While after the patch, we'll enqueue the
> new signal, and deliver the one that was already pending first.
>
> Can't really say whether the old behavior was necessary. At least
> the new behavior seems to make order or signal delivery consistent
> with how the order the signals were made pending in the
> first place, so seems better.
>
> This made me notice that this scenario of having more than one
> pending signal doesn't seem to be handled perfectly. We deliver
> the first signal, but nothing makes sure to get back control
> of the thread immediately in order to deliver the other pending
> signal, so it seems the thread may execute a bunch of arbitrary code
> until it next stops and is re-resumed, at which point we'll deliver
> the other pending signal. Maybe the simplest would be to
> force the thread to immediately stop again, by calling
> send_sigstop before resuming it, whenever we have more pending
> signals. Subject for a separate patch, in any case.

You meant "after resuming it" rather than "before resuming it", right? We have
two pending signals, so we resume the lwp and deliver the first signal. After
resuming, we need to immediately deliver the second signal, so we call
send_sigstop.

IIUC, this patch is OK as-is, right?

--
Yao (齐尧)