Date: Wed, 22 Jun 2011 18:51:11 +0300 From: Andriy Gapon <avg@FreeBSD.org> To: freebsd-current@FreeBSD.org, freebsd-stable@FreeBSD.org Subject: Re: [poll / rfc] kdb_stop_cpus Message-ID: <4E020F6F.7000502@FreeBSD.org> In-Reply-To: <4DE8FA2E.4030202@FreeBSD.org> References: <4DE8FA2E.4030202@FreeBSD.org>
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on 03/06/2011 18:13 Andriy Gapon said the following: > > I wonder if anybody uses kdb_stop_cpus with non-default value. I would like to go ahead and remove kdb_stop_cpus tunable/sysctl if nobody objects. > If, yes, I am very interested to learn about your usecase for it. > > I think that the default kdb behavior is the correct one, so it doesn't make sense > to have a knob to turn on incorrect behavior. > But I may be missing something obvious. > > The comment in the code doesn't really satisfy me: > /* > * Flag indicating whether or not to IPI the other CPUs to stop them on > * entering the debugger. Sometimes, this will result in a deadlock as > * stop_cpus() waits for the other cpus to stop, so we allow it to be > * disabled. In order to maximize the chances of success, use a hard > * stop for that. > */ > > The hard stop should be sufficiently mighty. > Yes, I am aware of supposedly extremely rare situations where a deadlock could > happen even when using hard stop. But I'd rather fix that than have this switch. > > Oh, the commit message (from 2004) explains it: >> Add a new sysctl, debug.kdb.stop_cpus, which controls whether or not we >> attempt to IPI other cpus when entering the debugger in order to stop >> them while in the debugger. The default remains to issue the stop; >> however, that can result in a hang if another cpu has interrupts disabled >> and is spinning, since the IPI won't be received and the KDB will wait >> indefinitely. We probably need to add a timeout, but this is a useful >> stopgap in the mean time. > > But that was before we started using hard stop in this context (in 2009). > -- Andriy Gapon
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