Date: Tue, 10 Nov 1998 01:47:08 +0100 (CET) From: Mikael Karpberg <karpen@ocean.campus.luth.se> To: brian@Awfulhak.org (Brian Somers) Cc: marcs@znep.com, brian@Awfulhak.org, freebsd-hackers@FreeBSD.ORG Subject: Re: bind()/listen() race Message-ID: <199811100047.BAA08580@ocean.campus.luth.se> In-Reply-To: <199811100007.AAA02529@woof.lan.awfulhak.org> from Brian Somers at "Nov 10, 98 00:07:38 am"
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According to Brian Somers:
> > You suggestion is possible as a workaround, and is probably the easiest
> > fix.
>
> It maybe the only practical one too. Backing off for a second and
> trying again will give us a situation where we can fail due to a
> previously crashed ppp (or maybe even resurrect the socket), or we
> get a connection.... We'd be extremely unlikely to have had a whole
> second between a bind() and listen() :-) If another ppp actually
> came up and went down again in that second, then we're probably going
> to do the same thing - due to authentication or IPCP negotiation
> failure (misconfiguration).
>
> I appreciate that some sort of elaborate read-lock-promoted
> -to-write-lock on the server side would allow the client to say ``if
> there's a write lock, connect(), otherwise if there's no read lock
> the server's dead - but this may be overkill.... The simpler the
> better.
Well... if you use a softlink (/var/run/mpppd.lock, or something?) which
points to the PID for the holder...
How about something like this?
int tmp, i_am_server = 0;
while ((tmp = create_softlink_to_pid()) == failure) {
if (kill_pid_with_signal_0() == success)
break;
remove_softlink(); /* lock was old */
}
if (tmp == failure) { /* we're not master */
tmp = 0;
while (connect() == failed && ++tmp < 10) {
sched_yield();
/* maybe add a "select(0,0,0,timeout);" with a small timeout too */
}
if (tmp == 10)
panic();
} else { /* we are master */
if (bind() = failed)
panic();
else
listen();
i_am_server = 1;
}
Wouldn't that maybe work?
Anyway, just brainstorming... Goodnight.
/Mikael
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