USB/coredump hangs in 8 and 9

Attilio Rao attilio at
Fri Aug 19 16:44:39 UTC 2011

2011/8/12 Andrew Boyer <aboyer at>:
> Re: panic: bufwrite: buffer is not busy??? (originally on freebsd-net)
> Re: debugging frequent kernel panics on 8.2-RELEASE (originally on freebsd-stable)
> Re: System hang in USB umass module while processing panic  (originally on freebsd-usb)
> Hello Andriy and Hans,
> Sorry for tying in so many discussions on this topic, but I think I have an explanation for the problems we have been reporting* with hanging coredumps on multicore systems on 8.2-RELEASE, and it has implications for Andriy's proposed scheduler patch** and for USB.
> In today's 8.X and 9.X branches, nothing that I can find stops the other CPUs when the kernel panics, but many parts of the locking code get disabled (grep on 'panicstr').  The 'bufwrite: buffer is not busy???' panic is caused by the syncer encountering an error.  If that happens when it's on the dumping CPU everything hangs.  If it's running on a different CPU, it will be blocked and hidden by the panic_cpu spinlock in panic(), and the dump continues, polling every attached keyboard for a Ctl-C.
> But, the new 8.X USB stack relies on multithreading.  (The new stack is the variable that broke coredumps for us in the 7.1->8.2 transition, I think.)  SVN 224223 fixes a hang that would happen when dumpsys() polls the USB keyboard (IPMI KVM, in our case).  That helps, but it only gets as far as usb_process(), where it hangs in a loop around a cv_wait() call.  This is easy to reproduce by adding code to the watchdog to break into the debugger if panicstr is set.
> I am experimenting with Andriy's patch** to stop the scheduler and it seems to be most of the way there, stopping the CPUs and disabling the rest of locking.  There are a few places that still reference panicstr, but that's minor.  These are the changes I made to the patch:
>  * Changed ukbd_do_poll() to return immediately if SCHEDULER_STOPPED() is true, so that we don't hang up in USB.  ukbd_yield()  locks up in DROP_GIANT(), and if you skip ukbd_yield(), usbd_transfer_poll() locks up trying to drop mutexes.
>  * Changed the call to spinlock_enter() back to critical_enter(), so that interrupts stay enabled and the hardclock still functions.

Which spinlock_enter() are you referring here?
I think that having interrupts fast handlers running during
panic/shutdown is something we should avoid like hell.


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