[PATCH] x86, kdump, ioapic: Fix kdump race with migrating irq

[Don Zickus]

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.

 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.

I attempted to resolve this by busting the spinlock in the kdump case only.
My justification was that kdump has already stopped the other cpus and it
is only clearing the io apic which shouldn't cause harm when overwriting
what the other cpu was doing.

I tested this by loading a dummy module that grabs the ioapic_lock and then
on another cpu, run 'echo c > /proc/sysrq-trigger'.  The deadlock was detected
and fixed with the patch below.

Signed-off-by: Don Zickus <dzickus at redhat.com>
---
 arch/x86/kernel/apic/io_apic.c     |   18 +++++++++++++++++-
 arch/x86/kernel/crash.c            |    2 +-
 arch/x86/kernel/machine_kexec_32.c |    2 +-
 arch/x86/kernel/machine_kexec_64.c |    2 +-
 arch/x86/kernel/reboot.c           |    2 +-
 5 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/apic/io_apic.c b/arch/x86/kernel/apic/io_apic.c
index fb07275..5fe4423 100644
--- a/arch/x86/kernel/apic/io_apic.c
+++ b/arch/x86/kernel/apic/io_apic.c
@@ -1991,9 +1991,25 @@ void __init enable_IO_APIC(void)
 /*
  * Not an __init, needed by the reboot code
  */
-void disable_IO_APIC(void)
+void disable_IO_APIC(int force)
 {
 	/*
+	 * Use force to bust the io_apic spinlock
+	 *
+	 * There is a case where kdump can race with irq
+	 * migration such that kdump will inject an NMI
+	 * while another cpu holds the ioapic_lock to
+	 * migrate the irq.  This would cause a deadlock.
+	 *
+	 * Because kdump stops all the cpus, we can safely
+	 * bust the spinlock as we are just clearing the
+	 * io apic anyway.
+	 */
+	if (force && spin_is_locked(&ioapic_lock))
+		/* only one cpu should be running now */
+		spin_lock_init(&ioapic_lock);
+
+	/*
 	 * Clear the IO-APIC before rebooting:
 	 */
 	clear_IO_APIC();
diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
index 13ad899..c8383b0 100644
--- a/arch/x86/kernel/crash.c
+++ b/arch/x86/kernel/crash.c
@@ -97,7 +97,7 @@ void native_machine_crash_shutdown(struct pt_regs *regs)
 
 	lapic_shutdown();
 #if defined(CONFIG_X86_IO_APIC)
-	disable_IO_APIC();
+	disable_IO_APIC(1);
 #endif
 #ifdef CONFIG_HPET_TIMER
 	hpet_disable();
diff --git a/arch/x86/kernel/machine_kexec_32.c b/arch/x86/kernel/machine_kexec_32.c
index a3fa43b..3c60005 100644
--- a/arch/x86/kernel/machine_kexec_32.c
+++ b/arch/x86/kernel/machine_kexec_32.c
@@ -212,7 +212,7 @@ void machine_kexec(struct kimage *image)
 		 * one form or other. kexec jump path also need
 		 * one.
 		 */
-		disable_IO_APIC();
+		disable_IO_APIC(0);
 #endif
 	}
 
diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
index b3ea9db..ed94a6a 100644
--- a/arch/x86/kernel/machine_kexec_64.c
+++ b/arch/x86/kernel/machine_kexec_64.c
@@ -295,7 +295,7 @@ void machine_kexec(struct kimage *image)
 		 * one form or other. kexec jump path also need
 		 * one.
 		 */
-		disable_IO_APIC();
+		disable_IO_APIC(0);
 #endif
 	}
 
diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
index 37a458b..766795c 100644
--- a/arch/x86/kernel/reboot.c
+++ b/arch/x86/kernel/reboot.c
@@ -652,7 +652,7 @@ void native_machine_shutdown(void)
 	lapic_shutdown();
 
 #ifdef CONFIG_X86_IO_APIC
-	disable_IO_APIC();
+	disable_IO_APIC(0);
 #endif
 
 #ifdef CONFIG_HPET_TIMER
-- 
1.7.7.5