Messages in this thread | | | Subject | Re: [BUG RT] dump-capture kernel not executed for panic in interrupt context | From | Joerg Vehlow <> | Date | Wed, 22 Jul 2020 06:30:53 +0200 |
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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,
Jörg
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@jv-coder.de> 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: >> #0 __rt_mutex_trylock (lock=0xffffffff81701aa0 <kexec_mutex>) at >> /usr/src/kernel/kernel/locking/rtmutex.c:2110 >> #1 0xffffffff8087601a in _mutex_trylock (lock=<optimised out>) at >> /usr/src/kernel/kernel/locking/mutex-rt.c:185 >> #2 0xffffffff803022a0 in __crash_kexec (regs=0x0 <irq_stack_union>) at >> /usr/src/kernel/kernel/kexec_core.c:941 >> #3 0xffffffff8027af59 in panic (fmt=0xffffffff80fa3d66 "softlockup: >> hung tasks") at /usr/src/kernel/kernel/panic.c:198 >> #4 0xffffffff80325b6d in watchdog_timer_fn (hrtimer=<optimised out>) at >> /usr/src/kernel/kernel/watchdog.c:464 >> #5 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 >> #6 __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 >> #7 0xffffffff802e7704 in hrtimer_interrupt (dev=<optimised out>) at >> /usr/src/kernel/kernel/time/hrtimer.c:1539 >> #8 0xffffffff80a020f2 in local_apic_timer_interrupt () at >> /usr/src/kernel/arch/x86/kernel/apic/apic.c:1067 >> #9 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. >> >> >> Jörg
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