[PATCH] x86/mce: Handle broadcasted MCE gracefully with kexec

[Borislav Petkov]

From: Xunlei Pang <xlpang@xxxxxxxxxx>

When we are about to kexec a crash kernel and right then and there a
broadcasted MCE fires while we're still in the first kernel and while
the other CPUs remain in a holding pattern, the #MC handler of the
first kernel will timeout and then panic due to never completing MCE
synchronization.

Handle this in a similar way as to when the CPUs are offlined when that
broadcasted MCE happens.

Suggested-by: Borislav Petkov <bp@xxxxxxxxx>
Signed-off-by: Xunlei Pang <xlpang@xxxxxxxxxx>
Acked-by: Tony Luck <tony.luck@xxxxxxxxx>
Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Cc: kexec@xxxxxxxxxxxxxxxxxxx
Cc: linux-edac <linux-edac@xxxxxxxxxxxxxxx>
Cc: x86-ml <x86@xxxxxxxxxx>
Link: http://lkml.kernel.org/r/1487857012-9059-1-git-send-email-xlpang@xxxxxxxxxx
[ Boris: rewrote commit message and comments. ]
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
---
 arch/x86/include/asm/reboot.h    |  1 +
 arch/x86/kernel/cpu/mcheck/mce.c | 18 ++++++++++++++++--
 arch/x86/kernel/reboot.c         |  5 +++--
 3 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/reboot.h b/arch/x86/include/asm/reboot.h
index 2cb1cc253d51..fc62ba8dce93 100644
--- a/arch/x86/include/asm/reboot.h
+++ b/arch/x86/include/asm/reboot.h
@@ -15,6 +15,7 @@ struct machine_ops {
 };
 
 extern struct machine_ops machine_ops;
+extern int crashing_cpu;
 
 void native_machine_crash_shutdown(struct pt_regs *regs);
 void native_machine_shutdown(void);
diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
index 8e9725c607ea..177472ace838 100644
--- a/arch/x86/kernel/cpu/mcheck/mce.c
+++ b/arch/x86/kernel/cpu/mcheck/mce.c
@@ -49,6 +49,7 @@
 #include <asm/tlbflush.h>
 #include <asm/mce.h>
 #include <asm/msr.h>
+#include <asm/reboot.h>
 
 #include "mce-internal.h"
 
@@ -1127,9 +1128,22 @@ void do_machine_check(struct pt_regs *regs, long error_code)
 	 * on Intel.
 	 */
 	int lmce = 1;
+	int cpu = smp_processor_id();
 
-	/* If this CPU is offline, just bail out. */
-	if (cpu_is_offline(smp_processor_id())) {
+	/*
+	 * Cases where we avoid rendezvous handler timeout:
+	 * 1) If this CPU is offline.
+	 *
+	 * 2) If crashing_cpu was set, e.g. we're entering kdump and we need to
+	 *  skip those CPUs which remain looping in the 1st kernel - see
+	 *  crash_nmi_callback().
+	 *
+	 * Note: there still is a small window between kexec-ing and the new,
+	 * kdump kernel establishing a new #MC handler where a broadcasted MCE
+	 * might not get handled properly.
+	 */
+	if (cpu_is_offline(cpu) ||
+	    (crashing_cpu != -1 && crashing_cpu != cpu)) {
 		u64 mcgstatus;
 
 		mcgstatus = mce_rdmsrl(MSR_IA32_MCG_STATUS);
diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
index e244c19a2451..d3718cc5edbf 100644
--- a/arch/x86/kernel/reboot.c
+++ b/arch/x86/kernel/reboot.c
@@ -749,10 +749,11 @@ void machine_crash_shutdown(struct pt_regs *regs)
 #endif
 
 
+/* This is the CPU performing the emergency shutdown work. */
+int crashing_cpu = -1;
+
 #if defined(CONFIG_SMP)
 
-/* This keeps a track of which one is crashing cpu. */
-static int crashing_cpu;
 static nmi_shootdown_cb shootdown_callback;
 
 static atomic_t waiting_for_crash_ipi;
-- 
2.11.0