Date:	Thu, 29 Mar 2012 12:02:27 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	x86@...nel.org
Cc:	LKML <linux-kernel@...r.kernel.org>,
	kexec-list <kexec@...ts.infradead.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH] x86, kdump: No need to disable ioapic in crash path

On Wed, Feb 29, 2012 at 03:08:49PM -0500, Don Zickus wrote:
> A customer of ours noticed when their machine crashed, kdump did not
> work but hung instead.  Using their firmware dumping solution they
> grabbed a vmcore and decoded the stacks on the cpus.  What they
> noticed seemed to be a rare deadlock with the ioapic_lock.

Any feedback?

Cheers,
Don

> 
>  CPU4:
>  machine_crash_shutdown
>  -> machine_ops.crash_shutdown
>     -> native_machine_crash_shutdown
>        -> kdump_nmi_shootdown_cpus ------> Send NMI to other CPUs
>        -> disable_IO_APIC
>           -> clear_IO_APIC
>              -> clear_IO_APIC_pin
>                 -> ioapic_read_entry
>                    -> spin_lock_irqsave(&ioapic_lock, flags)
>                    ---Infinite loop here---
> 
>  CPU0:
>  do_IRQ
>  -> handle_irq
>     -> handle_edge_irq
>         -> ack_apic_edge
>            -> move_native_irq
>                -> mask_IO_APIC_irq
>                   -> mask_IO_APIC_irq_desc
>                      -> spin_lock_irqsave(&ioapic_lock, flags)
>                      ---Receive NMI here after getting spinlock---
>                         -> nmi
>                            -> do_nmi
>                               -> crash_nmi_callback
>                               ---Infinite loop here---
> 
> The problem is that although kdump tries to shutdown minimal hardware,
> it still needs to disable the IO APIC.  This requires spinlocks which
> may be held by another cpu.  This other cpu is being held infinitely in
> an NMI context by kdump in order to serialize the crashing path.  Instant
> deadlock.
> 
> Eric, brought up a point that because the boot code was restructured we may
> not need to disable the io apic any more in the crash path.  The original
> concern that led to the development of disable_IO_APIC, was that the jiffies
> calibration on boot up relied on the PIT timer for reference.  Access
> to the PIT required 8259 interrupts to be working.  This wouldn't work
> if the ioapic needed to be configured.  So on panic path, the ioapic was
> reconfigured to use virtual wire mode to allow the 8259 to passthrough.
> 
> Those concerns don't hold true now, thanks to the jiffies calibration code
> not needing the PIT.  As a result, we can remove this call and simplify the
> locking needed in the panic path.
> 
> I tested kdump on an Ivy Bridge platform, a Pentium4 and an old athlon that
> did not have an ioapic.  All three were successful.
> 
> I also tested using lkdtm that would use jprobes to panic the system when
> entering do_IRQ.  The idea was to see how the system reacted with an
> interrupt pending in the second kernel.  My core2 quad successfully kdump'd
> 3 times in a row with no issues.
> 
> v2: removed the disable lapic code too
> v3: re-add disabling of lapic code
> 
> Cc: Eric W. Biederman <ebiederm@...ssion.com>
> Cc: Vivek Goyal <vgoyal@...hat.com>
> Signed-off-by: Don Zickus <dzickus@...hat.com>
> ---
> 
> There are really two problems here.  One is the deadlock of the ioapic_lock
> that I describe above.  Removing the code to disable the ioapic seems to
> resolve that.
> 
> The second issue is handling non-IRQ exceptions like NMIs.  Eric asked me
> to include removing the disable lapic code too.  However, because the nmi
> watchdog is stil active and kexec zeros out the idt before it jumps to
> purgatory, an NMI that comes in during the transition between the first
> kernel and second kernel will see an empty idt and reset the cpu.
> 
> Leaving the code to disable the lapic in, turns off perf and blocks those NMIs
> from happening (though an external NMI would still be an issue but that is no
> different than right now).
> 
> I tried playing with a stub idt and leaving it in place through the transition
> to the second kernel, but I can't quite get it to work correctly.  Spinning in the
> first kernel before the purgatory jump catches the idt properly.  Spinning in
> purgatory before the second kernel jump doesn't.  I even disabled the zero'ing
> out of the idt in the purgatory code.
> 
> I would like to get resolution on the ioapic deadlock to fix a customer issue
> while working the idt and NMI thing on the side, hence the split of this
> patchset.
> 
> Hopefully, people recognize there are two issues here and that this patch
> resolves the first one and the second one needs more debugging and time.
> ---
>  arch/x86/kernel/crash.c |    3 ---
>  1 files changed, 0 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> index 13ad899..b053cf9 100644
> --- a/arch/x86/kernel/crash.c
> +++ b/arch/x86/kernel/crash.c
> @@ -96,9 +96,6 @@ void native_machine_crash_shutdown(struct pt_regs *regs)
>  	cpu_emergency_svm_disable();
>  
>  	lapic_shutdown();
> -#if defined(CONFIG_X86_IO_APIC)
> -	disable_IO_APIC();
> -#endif
>  #ifdef CONFIG_HPET_TIMER
>  	hpet_disable();
>  #endif
> -- 
> 1.7.7.6
> 
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