Date: Sat, 03 Dec 2011 00:32:00 +0200 From: Andriy Gapon <avg@FreeBSD.org> To: John Baldwin <jhb@FreeBSD.org> Cc: Attilio Rao <attilio@FreeBSD.org>, freebsd-current@FreeBSD.org, Konstantin Belousov <kib@FreeBSD.org> Subject: Re: Stop scheduler on panic Message-ID: <4ED951E0.9000000@FreeBSD.org> In-Reply-To: <4ED91B8D.2080808@FreeBSD.org> References: <20111113083215.GV50300@deviant.kiev.zoral.com.ua> <201112011349.50502.jhb@freebsd.org> <4ED7E6B0.30400@FreeBSD.org> <201112011553.34432.jhb@freebsd.org> <4ED7F4BC.3080206@FreeBSD.org> <4ED855E6.20207@FreeBSD.org> <4ED8A306.9020801@FreeBSD.org> <4ED8F1C1.7010206@FreeBSD.org> <CAJ-FndCBXXGG%2BihS_rVfM5TqcopHABg80U0my9PxguYY8QBD=Q@mail.gmail.com> <4ED91B8D.2080808@FreeBSD.org>
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on 02/12/2011 20:40 John Baldwin said the following: > On 12/2/11 12:18 PM, Attilio Rao wrote: >> 2011/12/2 John Baldwin<jhb@freebsd.org>: >>> On 12/2/11 5:05 AM, Andriy Gapon wrote: >>>> >>>> on 02/12/2011 06:36 John Baldwin said the following: >>>>> >>>>> Ah, ok (I had thought SCHEDULER_STOPPED was going to always be true when >>>>> kdb was >>>>> active). But I think these two changes should cover critical_exit() ok. >>>>> >>>> >>>> I attempted to start a discussion about this a few times already :-) >>>> Should we treat kdb context the same as SCHEDULER_STOPPED context (in the >>>> current definition) ? That is, skip all locks in the same fashion? >>>> There are pros and contras. >>> >>> >>> kdb should not block on locks, no. Most debugger commands should not go >>> near locks anyway unless they are intended to carefully modify the existing >>> system in a safe manner (such as the 'kill' command which should only be >>> using try locks and fail if it cannot safely post the signal). >> >> The biggest problem to KDB as the same as panic is that doing proper >> 'continue' is impossible. >> One of the features of the 'skip-locking' path is that it doesn't take >> into account fast locking paths, where sometimes the lock can succeed >> and other fails and you don't know about them. Also the restarted CPUs >> can find corrupted datas (as they can be arbitrarely updated), I'm >> sure it is too much panic prone. > > Yes, my thought is that kdb commands, etc. should be using dedicated routines > that do not use locks whenever possible. The problem of a user > calling an arbitrary routine is not solvable (so I don't think we should try to > solve that, you use 'call' at your own risk), but built-in commands should > explicitly either 1) not use locking, or 2) only use try locks and fail out > cleanly (including dropping any try locks acquired) if a try fails. Now, that's > an ideal view, I don't know how close we are to that in practice or if it is a > realistically attainable goal. > I agree with what Attilio and you say. Initially it was tempting for me to apply the same SCHEDULER_STOPPED stopped medicine to the kdb_active context, but after trying to deal with kdb_active x SCHEDULER_STOPPED x ukbd situation I really changed my mind. I would classify the code that can be called in kdb_active context as follows: o debugger code proper (kdb, ddb, gdb stub, etc) - this obviously must not (doesn't have to) use any locking o code that can be invoked via 'call' command - this is essentially any code and I don't think that it can/should do anything special for the kdb_active context [*] o debugger helper routines - those that do something trivial should not acquire any locks; those that access shared resources should try the relevant locks and bail out if a resource can be in inconsistent state, or should be equipped to deal correctly with such a state; this is the same as what you say above o common code that the debuggers have to use - most obviously this is console code and drivers that serve a particular console; on one hand those drivers can have a non-trivial state that must be lock protected during normal operation, on the other hand the debugger must disregard those locks and grab its console; this is the most complex case in my opinion. Dealing with panics is much simpler, because it's a one way road to a system reset. Possibility to enter and exit debugger implies additional complications. So it doesn't look like SCHEDULER_STOPPED can be used equivalently for panic and for kdb_active. kdb_active requires more elaborate handling. [*] - but currently we depend on some "general purpose" routines to be 'callable' from debugger where we should really have a debugger command; the most popular example is 'call doadump'. -- Andriy Gapon
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