Commit ce4f06dc authored Jul 26, 2016 by Oleg Nesterov Committed by Ingo Molnar Jul 27, 2016

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@redhat.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dave Anderson <anderson@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
Link: http://lkml.kernel.org/r/20160726185736.GB4088@redhat.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 5cada174

kernel/stop_machine.c

+8 −0

Original line number	Diff line number	Diff line
		@@ -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);

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