Date: Thu, 16 Dec 2010 11:50:43 +0200 From: Andriy Gapon <avg@freebsd.org> To: perryh@pluto.rain.com Cc: freebsd-stable@freebsd.org Subject: Re: How to debug a double fault? (Re: Could MSGBUF_SIZE be made a loader tunable?) Message-ID: <4D09E0F3.5040302@freebsd.org> In-Reply-To: <4d09dd2e.iVncbZ/gHBXX0WdL%perryh@pluto.rain.com> References: <4cfc72a5.3nAjkv8mdrO/NrKQ%perryh@pluto.rain.com> <4CFD0633.9060509@freebsd.org> <4d089a74.vwMJkPEIddt7PIxy%perryh@pluto.rain.com> <4D08AACB.6060306@freebsd.org> <4d09dd2e.iVncbZ/gHBXX0WdL%perryh@pluto.rain.com>
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on 16/12/2010 11:34 perryh@pluto.rain.com said the following: > Andriy Gapon <avg@freebsd.org> wrote: >> on 15/12/2010 12:37 perryh@pluto.rain.com said the following: >>> Fatal double fault: >>> eip = 0xc07feb98 >>> esp = 0xc101e000 >>> ebp = 0xc101e004 >>> cpuid = 0; apic id = 00 >>> panic: double fault >>> cpuid = 0 >>> >>> How do I go about tracking this down? >> >> Do you have the standard debugging options in your kernel? > > No, it is 8.1-RELEASE GENERIC with only the name changed and the > (first attempt) msgbufsize patches applied. I was trying to > minimize changes to GENERIC, so as to minimize the opportunity > to screw something up, and I had this silly idea that something > this simple might "just work." > > It does occur to me to wonder whether any debugger would be > functional this early, before even the first line of the signon > message has been displayed. Is it possible, given the loader > messages, to come up with a base address which could be used to > compare the eip value with the kernel symbol table? Granted this > won't provide a traceback, but even knowing in which function it > crashed would be a start. You can research this approach, but I would just add KDB+DDB and get a stack trace without sweat. >> BTW, are you sure that you correctly placed initialization of >> msgbufsize ? > > I am not at all sure of that, and am not sufficiently familiar with > the sequence of events early in intiialization to know how to find > out -- although I suppose the observed crash might not be altogether > surprising if the kernel message buffer got allocated with a zero > size :( > > Apart from the name, msgbufsize is set up in exactly the same > way and place -- in init_param1() -- as maxswzone and maxbcache. > Perhaps that is not early enough; any idea what would be a better > example? I don't see any connection between msgbufsize and maxswzone, so I also don't know if that place is early enough. Just try to initialize the variable where it's defined and use TUNABLE_LONG. -- Andriy Gapon
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