Re: [BUG RT] dump-capture kernel not executed for panic in interrupt context

[Joerg Vehlow]

Hi Andrew,

it's been two month now and no reaction from you. Maybe you did not see 
this mail from Steven.
Please look at this issue.

Greets,

JÃrg

On 5/28/2020 2:46 PM, Steven Rostedt wrote:
> Hi Joerg,
>
> This does look like Andrew's commit (from 2008) is buggy (and this is a
> mainline bug, not an RT one). (top posting this so Andrew knows to look
> further ;-)
>
> On Thu, 28 May 2020 13:41:08 +0200
> Joerg Vehlow <lkml@xxxxxxxxxxx> wrote:
>
> > Hi,
> >
> > I think I found a bug in the kernel with rt patches (or maybe even without).
> > This applies to all kernels propably starting at 2.6.27.
> >
> > When a kernel panic is triggered from an interrupt handler, the dump-capture
> > kernel is not started, instead the system acts as if it was not installed.
> > The reason for this is, that panic calls __crash_kexec, which is protected
> > by a mutex. On an rt kernel this mutex is an rt mutex and when trylock
> > is called
> > on an rt mutex, the first check is whether the current kthread is in an
> > nmi or
> > irq handler. If it is, the function just returns 0 -> locking failed.
> >
> > According to rt_mutex_trylock documentation, it is not allowed to call this
> > function from an irq handler, but panic can be called from everywhere
> > and thus
> > rt_mutex_trylock can be called from everywhere. Actually even
> > mutex_trylock has
> > the comment, that it is not supposed to be used from interrupt context,
> > but it
> > still locks the mutex. I guess this could also be a bug in the non-rt
> > kernel.
> >
> > I found this problem using a test module, that triggers the softlock
> > detection.
> > It is a pretty simple module, that creates a kthread, that disables
> > preemption,
> > spins 60 seconds in an endless loop and then reenables preemption and
> > terminates
> > the thread. This reliably triggers the softlock detection and if
> > kernel.softlockup_panic=0, the system resumes perfectly fine afterwards. If
> > kernel.softlockup_panic=1 I would expect the dump-capture kernel to be
> > executed,
> > but it is not due to the bug (without rt patches it works), instead the
> > panic
> > function is executed until the end to the endless loop.
> >
> >
> > A stacktrace captured at the trylock call inside kexec_code looks like this:
> > #0Â __rt_mutex_trylock (lock=0xffffffff81701aa0 <kexec_mutex>) at
> > /usr/src/kernel/kernel/locking/rtmutex.c:2110
> > #1Â 0xffffffff8087601a in _mutex_trylock (lock=<optimised out>) at
> > /usr/src/kernel/kernel/locking/mutex-rt.c:185
> > #2Â 0xffffffff803022a0 in __crash_kexec (regs=0x0 <irq_stack_union>) at
> > /usr/src/kernel/kernel/kexec_core.c:941
> > #3Â 0xffffffff8027af59 in panic (fmt=0xffffffff80fa3d66 "softlockup:
> > hung tasks") at /usr/src/kernel/kernel/panic.c:198
> > #4Â 0xffffffff80325b6d in watchdog_timer_fn (hrtimer=<optimised out>) at
> > /usr/src/kernel/kernel/watchdog.c:464
> > #5Â 0xffffffff802e6b90 in __run_hrtimer (flags=<optimised out>,
> > now=<optimised out>, timer=<optimised out>, base=<optimised out>,
> > cpu_base=<optimised out>) at /usr/src/kernel/kernel/time/hrtimer.c:1417
> > #6Â __hrtimer_run_queues (cpu_base=0xffff88807db1c000, now=<optimised
> > out>, flags=<optimised out>, active_mask=<optimised out>) at
> > /usr/src/kernel/kernel/time/hrtimer.c:1479
> > #7Â 0xffffffff802e7704 in hrtimer_interrupt (dev=<optimised out>) at
> > /usr/src/kernel/kernel/time/hrtimer.c:1539
> > #8Â 0xffffffff80a020f2 in local_apic_timer_interrupt () at
> > /usr/src/kernel/arch/x86/kernel/apic/apic.c:1067
> > #9Â smp_apic_timer_interrupt (regs=<optimised out>) at
> > /usr/src/kernel/arch/x86/kernel/apic/apic.c:1092
> > #10 0xffffffff80a015df in apic_timer_interrupt () at
> > /usr/src/kernel/arch/x86/entry/entry_64.S:909
> >
> >
> > Obviously and as expected the panic was triggered in the context of the apic
> > interrupt. So in_irq() is true and trylock fails.
> >
> >
> > About 12 years ago this was not implemented using a mutex, but using xchg.
> > See: 8c5a1cf0ad3ac5fcdf51314a63b16a440870f6a2
> Yes, that commit is wrong, because mutex_trylock() is not to be taken in
> interrupt context, where crash_kexec() looks like it can be called.
>
> Unless back then crash_kexec() wasn't called in interrupt context, then the
> commit that calls it from that combined with this commit is the issue.
>
> -- Steve
>
> > Since my knowledege about mutexes inside the kernel is very limited, I
> > do not
> > know how this can be fixed and whether it should be fixed in the rt
> > patches or
> > if this really is a bug in mainline kernel (because trylock is also not
> > allowed
> > to be used in interrupt handlers.
> >
> >
> > JÃrg