Re: [PATCH 3/3] pids: Make it possible to clone tasks with given pids

[Pavel Emelyanov <xemul@parallels.com>]

>> This will make it impossible to fork() children on restore in parallel. And
>> I don't want to lose this ability :(
> 
> Yes, this is true. You need some form of synchronization in user-space.
> But, otoh, prctl/sysctl/whatever is much simpler. Both from implementation
> pov and from understanding/using. You can even do, say, pthread_create()
> to make a thread with the desired tid. And of course I like the fact we
> do not add the new hacks into copy_process's paths.
> 
> And. If you want to restore the process tree, then these new children
> have to cooperate anyway. Say, nobody can clone() without
> CLONE_CHILD_USEPIDS before we restore all pids.
> 
> Yes, sysctl+clone should be "atomic", but that is all. Does it really
> hurt? OK, if nothing else, can't you do somthing like
> 
> 	int fork_with_pid(int pid)
> 	{
> 		int ret;
> 		int pipefd[2];
> 
> 		pipe(pipefd);
> 
> 	retry:
> 		prcrl(PR_SET_LAST_PID, pid-1);
> 		ret = fork();
> 
> 		if (ret == 0) {
> 			/* child, wait from parent's ACK */
> 			read(pipefd[0], 1, NULL);
> 			return 0;
> 		}
> 
> 		/* raced with another user of PR_SET_LAST_PID */
> 		if (unlikely(ret != pid) {
> 			kill(ret, SIGKILL);
> 			waitpid(ret);
> 			goto retry;
> 		}
> 
> 		close(pipefd[1]);
> 		return pid;
> 	}
> 
> ?

Nope, as I said to Tejun, we will most likely not forks children in the depth-first
order, since tasks can share resources and we'll have to calculate the necessary fork
order. Thus this simple interaction simply won't work, more complexity will be required.

But I don't insist. If the CLONE_CHILD_USEPIDS has absolutely no way in the kernel we'll 
have to go the uglier path.

> Oleg.