On Wed, 2008-07-16 at 15:51 +0900, Hidetoshi Seto wrote:
quoted text
> If stop_machine() invoked while one of onlined cpu is locked up
> by some reason, stop_machine cannot finish its work because the
> locked cpu cannot stop.  This means all other healthy cpus
> will be blocked infinitely by one dead cpu.
> 
> This patch allows stop_machine to return -EBUSY with some printk
> messages if any of stop_machine's threads cannot start running on
> its target cpu in time.  You can enable this timeout via sysctl.
> 
> v3:
>  - set stopmachine_timeout default to 0 (= never timeout)
> 
> v2:
>  - remove fix for warning since it will be fixed upcoming typesafe
>    patches
>  - make stopmachine_timeout from secs to msecs
>  - allow disabling timeout by setting the stopmachine_timeout to 0
> 
> Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>

I really don't like this, it means the system is really screwed up and
doesn't deserve to continue.

quoted text
> ---
>  kernel/stop_machine.c |   54 ++++++++++++++++++++++++++++++++++++++++++++++--
>  kernel/sysctl.c       |   15 +++++++++++++
>  2 files changed, 66 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> index 5b72c2b..77b7944 100644
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -35,15 +35,18 @@ struct stop_machine_data {
>  };
>  
>  /* Like num_online_cpus(), but hotplug cpu uses us, so we need this. */
> -static unsigned int num_threads;
> +static atomic_t num_threads;
>  static atomic_t thread_ack;
> +static cpumask_t prepared_cpus;
>  static struct completion finished;
>  static DEFINE_MUTEX(lock);
>  
> +unsigned long stopmachine_timeout = 0; /* msecs, 0 = "never timeout" */
> +
>  static void set_state(enum stopmachine_state newstate)
>  {
>  	/* Reset ack counter. */
> -	atomic_set(&thread_ack, num_threads);
> +	atomic_set(&thread_ack, atomic_read(&num_threads));
>  	smp_wmb();
>  	state = newstate;
>  }
> @@ -67,6 +70,8 @@ static int stop_cpu(struct stop_machine_data *smdata)
>  	enum stopmachine_state curstate = STOPMACHINE_NONE;
>  	int uninitialized_var(ret);
>  
> +	cpu_set(smp_processor_id(), prepared_cpus);
> +
>  	/* Simple state machine */
>  	do {
>  		/* Chill out and ensure we re-read stopmachine_state. */
> @@ -90,6 +95,7 @@ static int stop_cpu(struct stop_machine_data *smdata)
>  		}
>  	} while (curstate != STOPMACHINE_EXIT);
>  
> +	atomic_dec(&num_threads);
>  	local_irq_enable();
>  	do_exit(0);
>  }
> @@ -105,6 +111,15 @@ int __stop_machine_run(int (*fn)(void *), void *data, const cpumask_t *cpus)
>  	int i, err;
>  	struct stop_machine_data active, idle;
>  	struct task_struct **threads;
> +	unsigned long limit;
> +
> +	if (atomic_read(&num_threads)) {
> +		/*
> +		 * previous stop_machine was timeout, and still there are some
> +		 * unfinished thread (dangling stucked CPU?).
> +		 */
> +		return -EBUSY;
> +	}
>  
>  	active.fn = fn;
>  	active.data = data;
> @@ -120,7 +135,7 @@ int __stop_machine_run(int (*fn)(void *), void *data, const cpumask_t *cpus)
>  	/* Set up initial state. */
>  	mutex_lock(&lock);
>  	init_completion(&finished);
> -	num_threads = num_online_cpus();
> +	atomic_set(&num_threads, num_online_cpus());
>  	set_state(STOPMACHINE_PREPARE);
>  
>  	for_each_online_cpu(i) {
> @@ -152,10 +167,21 @@ int __stop_machine_run(int (*fn)(void *), void *data, const cpumask_t *cpus)
>  
>  	/* We've created all the threads.  Wake them all: hold this CPU so one
>  	 * doesn't hit this CPU until we're ready. */
> +	cpus_clear(prepared_cpus);
>  	get_cpu();
>  	for_each_online_cpu(i)
>  		wake_up_process(threads[i]);
>  
> +	/* Wait all others come to life */
> +	if (stopmachine_timeout) {
> +		limit = jiffies + msecs_to_jiffies(stopmachine_timeout);
> +		while (cpus_weight(prepared_cpus) != num_online_cpus() - 1) {
> +			if (time_is_before_jiffies(limit))
> +				goto timeout;
> +			cpu_relax();
> +		}
> +	}
> +
>  	/* This will release the thread on our CPU. */
>  	put_cpu();
>  	wait_for_completion(&finished);
> @@ -169,10 +195,32 @@ kill_threads:
>  	for_each_online_cpu(i)
>  		if (threads[i])
>  			kthread_stop(threads[i]);
> +	atomic_set(&num_threads, 0);
>  	mutex_unlock(&lock);
>  
>  	kfree(threads);
>  	return err;
> +
> +timeout:
> +	printk(KERN_CRIT "stopmachine: Failed to stop machine in time(%lds).\n",
> +			stopmachine_timeout);
> +	for_each_online_cpu(i) {
> +		if (!cpu_isset(i, prepared_cpus) && i != smp_processor_id())
> +			printk(KERN_CRIT "stopmachine: cpu#%d seems to be "
> +					"stuck.\n", i);
> +		/* Unbind threads */
> +		set_cpus_allowed(threads[i], cpu_online_map);
> +	}
> +
> +	/* Let threads go exit */
> +	set_state(STOPMACHINE_EXIT);
> +
> +	put_cpu();
> +	/* no wait for completion */
> +	mutex_unlock(&lock);
> +	kfree(threads);
> +
> +	return -EBUSY;	/* canceled */
>  }
>  
>  int stop_machine_run(int (*fn)(void *), void *data, const cpumask_t *cpus)
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 2911665..3c7ca98 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -146,6 +146,10 @@ extern int no_unaligned_warning;
>  extern int max_lock_depth;
>  #endif
>  
> +#ifdef CONFIG_STOP_MACHINE
> +extern unsigned long stopmachine_timeout;
> +#endif
> +
>  #ifdef CONFIG_PROC_SYSCTL
>  static int proc_do_cad_pid(struct ctl_table *table, int write, struct file *filp,
>  		  void __user *buffer, size_t *lenp, loff_t *ppos);
> @@ -813,6 +817,17 @@ static struct ctl_table kern_table[] = {
>  		.child		= key_sysctls,
>  	},
>  #endif
> +#ifdef CONFIG_STOP_MACHINE
> +	{
> +		.ctl_name       = CTL_UNNUMBERED,
> +		.procname       = "stopmachine_timeout",
> +		.data           = &stopmachine_timeout,
> +		.maxlen         = sizeof(unsigned long),
> +		.mode           = 0644,
> +		.proc_handler   = &proc_doulongvec_minmax,
> +		.strategy       = &sysctl_intvec,
> +	},
> +#endif
>  /*
>   * NOTE: do not add new entries to this table unless you have read
>   * Documentation/sysctl/ctl_unnumbered.txt

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