Patch "x86/mce: Handle broadcasted MCE gracefully with kexec" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

x86/mce: Handle broadcasted MCE gracefully with kexec

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
x86-mce-handle-broadcasted-mce-gracefully-with-kexec.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From foo@baz Sun Mar 18 16:55:33 CET 2018
From: Xunlei Pang <[email protected]>
Date: Mon, 13 Mar 2017 10:50:19 +0100
Subject: x86/mce: Handle broadcasted MCE gracefully with kexec

From: Xunlei Pang <[email protected]>

[ Upstream commit 5bc329503e8191c91c4c40836f062ef771d8ba83 ]

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. ]

---
 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(-)

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/rtmutex-fix-pi-chain-order-integrity.patch queue-4.9/x86-mce-handle-broadcasted-mce-gracefully-with-kexec.patch --
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@@ -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);

@@ -48,6 +48,7 @@
 #include <asm/tlbflush.h>
 #include <asm/mce.h>
 #include <asm/msr.h>
+#include <asm/reboot.h>
 
 #include "mce-internal.h"
 

@@ -1081,9 +1082,22 @@ void do_machine_check(struct pt_regs *re
 	 * 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);

@@ -769,10 +769,11 @@ void machine_crash_shutdown(struct pt_re
 #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;