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    Subject[PATCH -mm] PM: Prevent frozen user mode helpers from failing the freezing of tasks (rev. 2)
    Date
    [I hope the ACKs still apply.]
    ---
    From: Rafael J. Wysocki <rjw@sisk.pl>

    At present, if a user mode helper is running while usermodehelper_pm_callback()
    is executed, the helper may be frozen and the completion in
    call_usermodehelper_exec() won't be completed until user space processes are
    thawed. As a result, the freezing of kernel threads may fail, which is not
    desirable.

    Prevent this from happening by introducing a counter of running user mode
    helpers and allowing usermodehelper_pm_callback() to succeed for
    action = PM_HIBERNATION_PREPARE or action = PM_SUSPEND_PREPARE only if there
    are no helpers running. [Namely, usermodehelper_pm_callback() waits for at most
    RUNNING_HELPERS_TIMEOUT for the number of running helpers to become zero and
    fails if that doesn't happen.]

    Special thanks to Uli Luckas <u.luckas@road.de>, Pavel Machek <pavel@ucw.cz> and
    Oleg Nesterov <oleg@tv-sign.ru> for reviewing the previous versions of this
    patch and for very useful comments.

    Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
    Acked-by: Uli Luckas <u.luckas@road.de>
    Acked-by: Nigel Cunningham <nigel@nigel.suspend2.net>
    Acked-by: Pavel Machek <pavel@ucw.cz>
    ---
    kernel/kmod.c | 78 ++++++++++++++++++++++++++++++++++++++++++++++++++--------
    1 file changed, 68 insertions(+), 10 deletions(-)

    Index: linux-2.6.22-rc4-mm2/kernel/kmod.c
    ===================================================================
    --- linux-2.6.22-rc4-mm2.orig/kernel/kmod.c
    +++ linux-2.6.22-rc4-mm2/kernel/kmod.c
    @@ -41,14 +41,6 @@ extern int max_threads;

    static struct workqueue_struct *khelper_wq;

    -/*
    - * If set, both call_usermodehelper_keys() and call_usermodehelper_pipe() exit
    - * immediately returning -EBUSY. Used for preventing user land processes from
    - * being created after the user land has been frozen during a system-wide
    - * hibernation or suspend operation.
    - */
    -static int usermodehelper_disabled;
    -
    #ifdef CONFIG_KMOD

    /*
    @@ -275,15 +267,55 @@ static void __call_usermodehelper(struct
    }
    }

    +#ifdef CONFIG_PM
    +/*
    + * If set, call_usermodehelper_exec() will exit immediately returning -EBUSY
    + * (used for preventing user land processes from being created after the user
    + * land has been frozen during a system-wide hibernation or suspend operation).
    + */
    +static int usermodehelper_disabled;
    +
    +/* Number of helpers running */
    +static atomic_t running_helpers = ATOMIC_INIT(0);
    +
    +/*
    + * Wait queue head used by usermodehelper_pm_callback() to wait for all running
    + * helpers to finish.
    + */
    +static DECLARE_WAIT_QUEUE_HEAD(running_helpers_waitq);
    +
    +/*
    + * Time to wait for running_helpers to become zero before the setting of
    + * usermodehelper_disabled in usermodehelper_pm_callback() fails
    + */
    +#define RUNNING_HELPERS_TIMEOUT (5 * HZ)
    +
    static int usermodehelper_pm_callback(struct notifier_block *nfb,
    unsigned long action,
    void *ignored)
    {
    + long retval;
    +
    switch (action) {
    case PM_HIBERNATION_PREPARE:
    case PM_SUSPEND_PREPARE:
    usermodehelper_disabled = 1;
    - return NOTIFY_OK;
    + smp_mb();
    + /*
    + * From now on call_usermodehelper_exec() won't start any new
    + * helpers, so it is sufficient if running_helpers turns out to
    + * be zero at one point (it may be increased later, but that
    + * doesn't matter).
    + */
    + retval = wait_event_timeout(running_helpers_waitq,
    + atomic_read(&running_helpers) == 0,
    + RUNNING_HELPERS_TIMEOUT);
    + if (retval) {
    + return NOTIFY_OK;
    + } else {
    + usermodehelper_disabled = 0;
    + return NOTIFY_BAD;
    + }
    case PM_POST_HIBERNATION:
    case PM_POST_SUSPEND:
    usermodehelper_disabled = 0;
    @@ -293,6 +325,30 @@ static int usermodehelper_pm_callback(st
    return NOTIFY_DONE;
    }

    +static void helper_lock(void)
    +{
    + atomic_inc(&running_helpers);
    + smp_mb__after_atomic_inc();
    +}
    +
    +static void helper_unlock(void)
    +{
    + if (atomic_dec_and_test(&running_helpers))
    + wake_up(&running_helpers_waitq);
    +}
    +
    +static void register_pm_notifier_callback(void)
    +{
    + pm_notifier(usermodehelper_pm_callback, 0);
    +}
    +#else /* CONFIG_PM */
    +#define usermodehelper_disabled 0
    +
    +static inline void helper_lock(void) {}
    +static inline void helper_unlock(void) {}
    +static inline void register_pm_notifier_callback(void) {}
    +#endif /* CONFIG_PM */
    +
    /**
    * call_usermodehelper_setup - prepare to call a usermode helper
    * @path - path to usermode executable
    @@ -397,6 +453,7 @@ int call_usermodehelper_exec(struct subp
    DECLARE_COMPLETION_ONSTACK(done);
    int retval;

    + helper_lock();
    if (sub_info->path[0] == '\0') {
    retval = 0;
    goto out;
    @@ -418,6 +475,7 @@ int call_usermodehelper_exec(struct subp

    out:
    call_usermodehelper_freeinfo(sub_info);
    + helper_unlock();
    return retval;
    }
    EXPORT_SYMBOL(call_usermodehelper_exec);
    @@ -459,5 +517,5 @@ void __init usermodehelper_init(void)
    {
    khelper_wq = create_singlethread_workqueue("khelper");
    BUG_ON(!khelper_wq);
    - pm_notifier(usermodehelper_pm_callback, 0);
    + register_pm_notifier_callback();
    }
    -
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