how to continue from a breakpoint in GNU/Hurd

Hi.

I am writing a debugger demo under GNU/Hurd when I study the gdb
source code. And I have met a big problem that when I set a breakpoint
in the inferior, my code can't resume inferior from it.

I can get the exception message from the mach kernel when the inferior
hit the breakpoint. As soon as I got the exception message, I removed
the int3, and set the EIP to the previous values, then post a signal
zero to the inferior by msg_sig_post_untraced_request(). at last I
called thread_resume() to resume it. But thing goes wrong, the
inferior is always stops at the breakpoint. and seems never to
continue.

I don't know how to fix this, could any of you give me a suggestion?
thanks very much.

--
Yue Lu (陆岳)