Date: Tue, 12 Nov 2013 13:51:51 -0800 From: Justin Hibbits <jhibbits@freebsd.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: FreeBSD PowerPC ML <freebsd-ppc@freebsd.org> Subject: Re: Strange panic on ppc64 Message-ID: <CAHSQbTA8vCgiYwxDQ7J-w4K_vXEUr2a20tLYOBQXLq=cb=OggQ@mail.gmail.com> In-Reply-To: <20131112214655.GZ59496@kib.kiev.ua> References: <CAHSQbTD6%2BDd-So88gSArTtpcA=w4D-GibGpoFLoHQuFPjUrKuA@mail.gmail.com> <20131112205142.GY59496@kib.kiev.ua> <CAHSQbTCnFnVBtL%2B9VOn%2B9zNMJqt=cLyKB6AYjDzjHrddZq65ug@mail.gmail.com> <20131112214655.GZ59496@kib.kiev.ua>
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On Nov 12, 2013 1:47 PM, "Konstantin Belousov" <kostikbel@gmail.com> wrote: > > On Tue, Nov 12, 2013 at 01:13:28PM -0800, Justin Hibbits wrote: > > On Tue, Nov 12, 2013 at 12:51 PM, Konstantin Belousov > > <kostikbel@gmail.com>wrote: > > > > > On Tue, Nov 12, 2013 at 08:32:31AM -0800, Justin Hibbits wrote: > > > > The log is attached. I'm not sure what exactly is going on here. The > > > > conditions were: building something on zfs, while also accessing files > > > over > > > > NFS. It seems each of those individually is fine, but doing both it > > > brings > > > > my system down. I _think_ the actual panic message (recursed on > > > > non-recursive mutex) is a red herring, since it already trapped in the > > > > kernel, twice. Any clues? It's 100% reproducible by me. > > > > > > > This does not seems related to NFS or ZFS proper. What happens is > > > that tc_windup() executing in the interupt context decided to enter > > > a debugger. I am not sure why the debugger is entered. > > > > > > Apart from this, the situation is clear: > > > the interrupt happens while the referenced mutex was owned. The debugger > > > is entered, and tries to read a char from keyboard, which is USB. For > > > USB to function, it has to access a lot of the kernel services, in > > > particular, busdma, which, in turn, requires some pmap calls, and you > > > end up accessing the same mutex. > > > > > > The bug there is that code executed from interrupt or debugger context > > > must not lock mutexes, or generally, call into top-half of the kernel > > > (now top half is essentially the whole kernel). I am not sure if > > > USB could ever work in such mode. > > > > > > > I discussed this with Nathan on IRC earlier. You're right that it's not > > related to NFS nor ZFS, at least not directly. It's actually most likely a > > stack overflow, since currently there are only 4 pages for stack, so when > > it takes the DECR trap it ends up blowing the stack. This is only made > > evident because ZFS is very stack hungry. I'm upping the stack to 8 pages, > > and testing tonight. > > > > As for your assessment of the situation, you're spot on, and I have no idea > > how to properly fix it. > > For stack overflow, I would not see the frames I talked about. > The panic clearly states that you get a recursion on mutex, and sleepable > mutex must not be locked from the interrupt or debugger context. Right, it is two issues. It entered the debugger with a stack overflow, then panicked on a mutex recursion. I'm addressing the stack overflow.
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