x86/mce: Handle broadcasted MCE gracefully with kexec

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.

[ Boris: rewrote commit message and comments. ]
Suggested-by: NBorislav Petkov <bp@alien8.de>
Signed-off-by: NXunlei Pang <xlpang@redhat.com>
Signed-off-by: NBorislav Petkov <bp@suse.de>
Acked-by: NTony Luck <tony.luck@intel.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: kexec@lists.infradead.org
Cc: linux-edac <linux-edac@vger.kernel.org>
Link: http://lkml.kernel.org/r/1487857012-9059-1-git-send-email-xlpang@redhat.com
Link: http://lkml.kernel.org/r/20170313095019.19351-1-bp@alien8.deSigned-off-by: NThomas Gleixner <tglx@linutronix.de>

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);

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);

arch/x86/kernel/reboot.c

@@ -765,10 +765,11 @@ 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;