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Subjectwaitpid(WNOHANG) should report SIGCHLD-notified signals [Re: [PATCH 09/11] job control: reorganize wait_task_stopped()]
On Thu, 12 May 2011 19:32:28 +0200, Tejun Heo wrote:
[...]
> while (1) {
> siginfo_t si = {};
>
> waitid(P_PID, child, &si,
> WSTOPPED | WCONTINUED | WNOWAIT | WNOHANG);
> if (!si.si_pid)
> break;
> }
>
> kill(control, SIGKILL);
> kill(child, SIGKILL);
> return 0;
> }
>
> waitid(2) should always succeed as it's never consuming wait state,
> but it does, with or without the patch. All transitions need to be
> made water tight to remove the bug.

It may be not related, I do not inderstand the kernel internals being
discussed.

I was told by Roland McGrath that after SIGCHLD of a ptrace even from tracee
invokes sighandler for that SIGCHLD in the tracer then waitpid(WNOHANG) still
may return 0 as the signal is not yet ready. I was not able to reproduce it
with a testcase myself.

But if it is a case it should be fixed as there is no later notification when
to call waitpid(WNOHANG) again. And sure the debugger cannot busy-loop poll
it.


Thanks,
Jan


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