Patch "x86/mce: Ensure offline CPUs don't participate in rendezvous process" has been added to the 4.3-stable tree

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

x86/mce: Ensure offline CPUs don't participate in rendezvous process

to the 4.3-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-ensure-offline-cpus-don-t-participate-in-rendezvous-process.patch
and it can be found in the queue-4.3 subdirectory.

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

From: Ashok Raj <[email protected]>
Date: Thu, 10 Dec 2015 11:12:26 +0100
Subject: x86/mce: Ensure offline CPUs don't participate in rendezvous process

From: Ashok Raj <[email protected]>

commit d90167a941f62860f35eb960e1012aa2d30e7e94 upstream.

Intel's MCA implementation broadcasts MCEs to all CPUs on the node. This poses a problem for offlined CPUs which cannot participate in the rendezvous process:

  Kernel panic - not syncing: Timeout: Not all CPUs entered broadcast exception handler
  Kernel Offset: disabled
  Rebooting in 100 seconds..

More specifically, Linux does a soft offline of a CPU when writing a 0 to /sys/devices/system/cpu/cpuX/online, which doesn't prevent the #MC exception from being broadcasted to that CPU.

Ensure that offline CPUs don't participate in the MCE rendezvous and clear the RIP valid status bit so that a second MCE won't cause a shutdown.

Without the patch, mce_start() will increment mce_callin and wait for all CPUs. Offlined CPUs should avoid participating in the rendezvous process altogether.

---
 arch/x86/kernel/cpu/mcheck/mce.c |   11 +++++++++++
 1 file changed, 11 insertions(+)

@@ -999,6 +999,17 @@ void do_machine_check(struct pt_regs *re
 	int flags = MF_ACTION_REQUIRED;
 	int lmce = 0;
 
+	/* If this CPU is offline, just bail out. */
+	if (cpu_is_offline(smp_processor_id())) {
+		u64 mcgstatus;
+
+		mcgstatus = mce_rdmsrl(MSR_IA32_MCG_STATUS);
+		if (mcgstatus & MCG_STATUS_RIPV) {
+			mce_wrmsrl(MSR_IA32_MCG_STATUS, 0);
+			return;
+		}
+	}
+
 	ist_enter(regs);
 
 	this_cpu_inc(mce_exception_count);

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

queue-4.3/x86-mce-ensure-offline-cpus-don-t-participate-in-rendezvous-process.patch