linux-kernel - Re: [RESEND][RFC PATCH v2] waitfd

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Message-ID: <alpine.DEB.1.10.0901101454110.22208@alien.or.mcafeemobile.com>
Date:	Sat, 10 Jan 2009 15:11:28 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Scott James Remnant <scott@...onical.com>
cc:	Oleg Nesterov <oleg@...hat.com>,
	Roland McGrath <roland@...hat.com>,
	Ingo Molnar <mingo@...e.hu>, Casey Dahlin <cdahlin@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [RESEND][RFC PATCH v2] waitfd

On Sat, 10 Jan 2009, Scott James Remnant wrote:

> First, what we have today:
> 
> 	sigemptyset (&mask);
> 	sigaddset (&mask, SIGCHLD);
> 
> 	/* Block normal delivery and receive by sigfd instead */
> 	sigprocmask (SIG_BLOCK, &mask, NULL);
> 	sigfd = signalfd (-1, &mask, 0);
> 
> 	for (;;) {
> 		read (sigfd, &fd_siginfo, sizeof siginfo);
> 
> 		/* throw away fd_siginfo, we're reading SIGCHLD and
> 		 * can't use it :-(
> 		 */
> 
> 		/* SIGCHLD means _at_least_one_ child is pending, there
> 		 * may be more; so we have to loop AND expect to find
> 		 * nothing
> 		 */
> 		for (;;) {
> 			/* ARGH! waitid returns 0 with WNOHANG if there
> 			 * are no children.
> 			 *
> 			 * AND the structure, despite being logically
> 			 * the same, isn't the same as the signalfd
> 			 * one :-/
> 			 */
> 			memset (&w_siginfo, 0, sizeof w_siginfo);
> 
> 			waitid (P_ALL, 0, &w_siginfo,
> 				WEXITED | WNOHANG);
> 
> 			/* Did we find anything? */
> 			if (! w_siginfo.si_pid)
> 				break;
> 
> 			/* NOW we have the siginfo_t for a recently
> 			 * deceased process
> 			 */
> 
> 			mourn (&w_siginfo);
> 		}
> 

That, once all the glamorous comments are removed, can be solved pretty 
much with this much userspace code:

int waitfd(int flags) {
	sigemptyset(&mask);
	sigaddset(&mask, SIGCHLD);
	sigprocmask(SIG_BLOCK, &mask, NULL);

	return signalfd(-1, &mask, flags);
}

int waitfd_read(int fd, siginfo_t *si) {
	for (;;) {
		if (read(sigfd, &fd_siginfo,
		    sizeof fd_siginfo) != sizeof(fd_siginfo)
			retrun 0;
		memset(si, 0, sizeof *si);
		waitid(P_ALL, 0, si, WEXITED | WNOHANG);
		if (si->si_pid)
			break;
	}
	return sizeof *si;
}


About the exit_signal, that would have made probably more sense for 
it to be a parent property, instead of childs one. And be:

	do_notify_parent(p, p->parent->exit_signal);

instead of:

	do_notify_parent(p, p->exit_signal);

After all, is the parent that is going to make use of the notification, 
not the child.



- Davide


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