On Tue, 26 Jul 2016, Oleg Nesterov wrote:
> 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 <shuw...@redhat.com>
> Signed-off-by: Oleg Nesterov <o...@redhat.com>
Reviewed-by: Thomas Gleixner <t...@linutronix.de>
> ---
> 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);
>
> --
> 2.5.0
>
>
>
