Re: [PATCH] x86, kdump: No need to disable ioapic in crash path

[Don Zickus]

On Thu, Mar 15, 2012 at 05:16:50PM -0400, Seiji Aguchi wrote:
> Don,
> 
> What do you think about following scenario?
> Disabling I/O APIC seems to be needed before booting kdump kernel.

This patch was for *booting* the second kernel.  Kexec/kdump disables
interrupts before jumping into the second kernel, so it doesn't matter
what is done with the i/o apic as interrupts are blocked until the second
kernel boots up and deals with them.

It is Suresh's patch that helps enable a patch like the one I proposed.

Cheers,
Don

> 
> Seiji
> 
> 
> commit 1e75b31d638d5242ca8e9771dfdcbd28a5f041df
> Author: Suresh Siddha <suresh.b.siddha@xxxxxxxxx>
> Date:   Thu Aug 25 12:01:11 2011 -0700
> 
>     x86, kdump, ioapic: Reset remote-IRR in clear_IO_APIC
>     
>     In the kdump scenario mentioned below, we can have a case where
>     the device using level triggered interrupt will not generate any
>     interrupts in the kdump kernel.
>     
>     1. IO-APIC sends a level triggered interrupt to the CPU's local APIC.
>     
>     2. Kernel crashed before the CPU services this interrupt, leaving
>        the remote-IRR in the IO-APIC set.
>     
>     3. kdump kernel boot sequence does clear_IO_APIC() as part of IO-APIC
>        initialization. But this fails to reset remote-IRR bit of the
>        IO-APIC RTE as the remote-IRR bit is read-only.
>     
>     4. Device using that level triggered entry can't generate any
>        more interrupts because of the remote-IRR bit.
>     
>     In clear_IO_APIC_pin(), check if the remote-IRR bit is set and if
>     so do an explicit attempt to clear it (by doing EOI write on
>     modern io-apic's and changing trigger mode to edge/level on
>     older io-apic's). Also before doing the explicit EOI to the
>     io-apic, ensure that the trigger mode is indeed set to level.
>     This will enable the explicit EOI to the io-apic to reset the
>     remote-IRR bit.
>     
>     Tested-by: Leonardo Chiquitto <lchiquitto@xxxxxxxxxx>
>     Signed-off-by: Suresh Siddha <suresh.b.siddha@xxxxxxxxx>
>     Fixes: https://bugzilla.novell.com/show_bug.cgi?id=701686
>     Cc: Rafael Wysocki <rjw@xxxxxxxxxx>
>     Cc: Maciej W. Rozycki <macro@xxxxxxxxxxxxxx>
>     Cc: Thomas Renninger <trenn@xxxxxxx>
>     Cc: jbeulich@xxxxxxxxxx
>     Cc: yinghai@xxxxxxxxxx
>     Link: http://lkml.kernel.org/r/20110825190657.157502602@xxxxxxxxxxxxxxxxxxxxxxxxxx
>     Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
> 
> > -----Original Message-----
> > From: linux-kernel-owner@xxxxxxxxxxxxxxx [mailto:linux-kernel-owner@xxxxxxxxxxxxxxx] On Behalf Of Don Zickus
> > Sent: Thursday, March 15, 2012 4:27 PM
> > To: x86@xxxxxxxxxx
> > Cc: LKML; kexec-list; Eric W. Biederman; Vivek Goyal
> > 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.
> > 
> > While we are discussing the NMI stuff in another thread, does anyone have any objection to committing this patch.  It fixes a real
> > problem today.
> > 
> > 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@xxxxxxxxxxxx>
> > > Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
> > > Signed-off-by: Don Zickus <dzickus@xxxxxxxxxx>
> > > ---
> > >
> > > 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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