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    Subject[PATCH] stopmachine: add stopmachine_timeout v2
    Thank you for useful feedbacks!
    Here is the updated version.
    Could you put this on top of your patches, Rusty?

    Thanks,
    H.Seto


    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.

    v2:
    - remove fix for warning since it will be fixed upcoming typesafe
    patches
    - make stopmachine_timeout from secs to msecs, and set default to
    200 msec (since v1's arbitrary 5 sec is too long)
    - allow disabling timeout by setting the stopmachine_timeout to 0

    Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
    ---
    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..2968b8a 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 = 200; /* msecs, arbitrary */
    +
    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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