linux-kernel - [PATCH] x86/mce: Ensure offline CPUs don't participate in rendezvous process

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Date:	Thu, 10 Dec 2015 11:12:26 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH] x86/mce: Ensure offline CPUs don't participate in rendezvous process

From: Ashok Raj <ashok.raj@...el.com>

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.

Reviewed-by: Tony Luck <tony.luck@...el.com>
Signed-off-by: Ashok Raj <ashok.raj@...el.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: linux-edac <linux-edac@...r.kernel.org>
Cc: <stable@...r.kernel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: x86-ml <x86@...nel.org>
Link: http://lkml.kernel.org/r/1449188170-3909-1-git-send-email-ashok.raj@intel.com
[ Massage commit message. ]
Signed-off-by: Borislav Petkov <bp@...e.de>
---
 arch/x86/kernel/cpu/mcheck/mce.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
index c5b0d562dbf5..7e8a736d09db 100644
--- a/arch/x86/kernel/cpu/mcheck/mce.c
+++ b/arch/x86/kernel/cpu/mcheck/mce.c
@@ -999,6 +999,17 @@ void do_machine_check(struct pt_regs *regs, long error_code)
 	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);
-- 
2.3.5

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