Re: [PATCH 1/1] stop_machine: touch_nmi_watchdog() after MULTI_STOP_PREPARE

[Thomas Gleixner]

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 <shuwang@xxxxxxxxxx>
> Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>

Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

> ---
>  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
> 
> 
>