Re: [PATCH] Fix pldd not to leave process stopped after detaching

[Pedro Alves <palves at redhat dot com>]

On 08/27/2013 11:44 AM, Andreas Schwab wrote:
> When attaching to a process we always have to wait for the tracee to
> enter ptrace-stop state.  Otherwise this can result in a race where the
> STOP signal is reveived after we have already detached from it again,
> leaving the process in stopped state.

Won't this hang with job stopped processes?

gdb has this to handle that scenario (linux_nat_post_attach_wait):

  if (linux_proc_pid_is_stopped (pid))  // this is a /proc/PID/status check
    {
      if (debug_linux_nat)
	fprintf_unfiltered (gdb_stdlog,
			    "LNPAW: Attaching to a stopped process\n");

      /* The process is definitely stopped.  It is in a job control
	 stop, unless the kernel predates the TASK_STOPPED /
	 TASK_TRACED distinction, in which case it might be in a
	 ptrace stop.  Make sure it is in a ptrace stop; from there we
	 can kill it, signal it, et cetera.

         First make sure there is a pending SIGSTOP.  Since we are
	 already attached, the process can not transition from stopped
	 to running without a PTRACE_CONT; so we know this signal will
	 go into the queue.  The SIGSTOP generated by PTRACE_ATTACH is
	 probably already in the queue (unless this kernel is old
	 enough to use TASK_STOPPED for ptrace stops); but since SIGSTOP
	 is not an RT signal, it can only be queued once.  */
      kill_lwp (pid, SIGSTOP);

      /* Finally, resume the stopped process.  This will deliver the SIGSTOP
	 (or a higher priority signal, just like normal PTRACE_ATTACH).  */
      ptrace (PTRACE_CONT, pid, 0, 0);
    }

-- 
Pedro Alves