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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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