Date: Fri, 03 Jun 2011 18:13:50 +0300 From: Andriy Gapon <avg@FreeBSD.org> To: freebsd-current@FreeBSD.org, freebsd-stable@FreeBSD.org Subject: [poll / rfc] kdb_stop_cpus Message-ID: <4DE8FA2E.4030202@FreeBSD.org>
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I wonder if anybody uses kdb_stop_cpus with non-default value. 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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