linux-kernel - [tip:core/urgent] stop_machine: Touch_nmi_watchdog() after MULTI_STOP

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Date:	Wed, 27 Jul 2016 03:42:50 -0700
From:	tip-bot for Oleg Nesterov <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	paulmck@...ux.vnet.ibm.com, oleg@...hat.com, tj@...nel.org,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	shuwang@...hat.com, peterz@...radead.org, tglx@...utronix.de,
	mingo@...nel.org, hpa@...or.com, anderson@...hat.com,
	torvalds@...ux-foundation.org
Subject: [tip:core/urgent] stop_machine: Touch_nmi_watchdog() after
 MULTI_STOP_PREPARE

Commit-ID:  ce4f06dcbb5d6d04d202f1b81ac72d5679dcdfc0
Gitweb:     http://git.kernel.org/tip/ce4f06dcbb5d6d04d202f1b81ac72d5679dcdfc0
Author:     Oleg Nesterov <oleg@...hat.com>
AuthorDate: Tue, 26 Jul 2016 20:57:36 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 27 Jul 2016 11:12:11 +0200

stop_machine: Touch_nmi_watchdog() after MULTI_STOP_PREPARE

Suppose that stop_machine(fn) hangs because fn() hangs. In this case NMI
hard-lockup can be triggered on another CPU which does nothing wrong and
the trace from nmi_panic() won't help to investigate the problem.

And this change "fixes" the problem we (seem to) hit in practice.

 - stop_two_cpus(0, 1) races with show_state_filter() running on CPU_0.

 - CPU_1 already spins in MULTI_STOP_PREPARE state, it detects the soft
   lockup and tries to report the problem.

 - show_state_filter() enables preemption, CPU_0 calls multi_cpu_stop()
   which goes to MULTI_STOP_DISABLE_IRQ state and disables interrupts.

 - CPU_1 spends more than 10 seconds trying to flush the log buffer to
   the slow serial console.

 - NMI interrupt on CPU_0 (which now waits for CPU_1) calls nmi_panic().

Reported-by: Wang Shu <shuwang@...hat.com>
Signed-off-by: Oleg Nesterov <oleg@...hat.com>
Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Dave Anderson <anderson@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Tejun Heo <tj@...nel.org>
Link: http://lkml.kernel.org/r/20160726185736.GB4088@redhat.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/stop_machine.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index a467e6c..4a1ca5f 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -21,6 +21,7 @@
 #include <linux/smpboot.h>
 #include <linux/atomic.h>
 #include <linux/lglock.h>
+#include <linux/nmi.h>
 
 /*
  * Structure to determine completion condition and record errors.  May
@@ -209,6 +210,13 @@ static int multi_cpu_stop(void *data)
 				break;
 			}
 			ack_state(msdata);
+		} else if (curstate > MULTI_STOP_PREPARE) {
+			/*
+			 * At this stage all other CPUs we depend on must spin
+			 * in the same loop. Any reason for hard-lockup should
+			 * be detected and reported on their side.
+			 */
+			touch_nmi_watchdog();
 		}
 	} while (curstate != MULTI_STOP_EXIT);