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    SubjectRe: freezer problems
    Date
    On Tuesday, 20 February 2007 01:32, Rafael J. Wysocki wrote:
    > On Tuesday, 20 February 2007 01:12, Oleg Nesterov wrote:
    > > On 02/20, Rafael J. Wysocki wrote:
    > > >
    > > > On Monday, 19 February 2007 23:41, Oleg Nesterov wrote:
    > > > > On 02/19, Rafael J. Wysocki wrote:
    > > > > >
    > > > > > On Monday, 19 February 2007 21:23, Oleg Nesterov wrote:
    > > > > >
    > > > > > > > @@ -199,6 +189,10 @@ static void thaw_tasks(int thaw_user_spa
    > > > > > > >
    > > > > > > > do_each_thread(g, p) {
    > > > > > > > + if (freezer_should_skip(p))
    > > > > > > > + cancel_freezing(p);
    > > > > > > > + } while_each_thread(g, p);
    > > > > > > > + do_each_thread(g, p) {
    > > > > > > > if (!freezeable(p))
    > > > > > > > continue;
    > > > > > >
    > > > > > > Any reason for 2 separate do_each_thread() loops ?
    > > > > >
    > > > > > Yes. If there is a "freeze" request pending for the vfork parent (TIF_FREEZE
    > > > > > set), we have to cancel it before the child is unfrozen, since otherwise the
    > > > > > parent may go freezing after we try to reset PF_FROZEN for it.
    > > > >
    > > > > I see, thanks... thaw_process() doesn't take TIF_FREEZE into account.
    > > > >
    > > > > But doesn't this mean we have a race?
    > > > >
    > > > > Suppose that try_to_freeze_tasks() failed. It does cancel_freezing() for each
    > > > > process before return, but what if some thread already checked TIF_FREEZE and
    > > > > (for simplicity) it is preempted before frozen_process() in refrigerator().
    > > > >
    > > > > thaw_tasks() runs, ignores this task (P), returns. P gets CPU, and becomes
    > > > > frozen, but nobody will thaw it.
    > > > >
    > > > > No?
    > > >
    > > > Well, I think this is highly theoretical. Namely, try_to_freeze_tasks() only
    > > > fails after the timeout that's currently set to 20 sec., and it yields the CPU
    > > > in each iteration of the main loop. The task in question would have to refuse
    > > > being frozen for 20 sec. and then suddenly decide to freeze itself right before
    > > > try_to_freeze_tasks() checks the timeout for the very last time. Then, it
    > > > would have to get preempted at this very moment and stay unfrozen at least
    > > > until thaw_tasks() starts running and in fact even longer.
    > >
    > > Yes, yes, it is pure theroretical,
    > >
    > > > I think we may avoid this by making try_to_freeze_tasks() sleep for some time
    > > > after it has reset TIF_FREEZE for all tasks in the error path, if anyone is
    > > > ever able to trigger it.
    > >
    > > This makes this race (pure theroretical) ** 2 :)
    > >
    > > Still. May be it make sense to introduce cancel_freezing_and_thaw() function
    > > (not right now) which stops the task from sleeping in refrigirator reliably.
    >
    > Hm. In the case discussed above we have a task that's right before calling
    > frozen_process(), so we can't thaw it, because it's not frozen. It will be
    > frozen just in a while, but try_to_freeze_tasks() and thaw_tasks() have no
    > way to check this.
    >
    > I think to close this race the refrigerator should check TIF_FREEZE and set
    > PF_FROZEN _and_ reset TIF_FREEZE under a lock that would also have to be
    > taken by try_to_freeze_tasks() in the beginning of the error path. This will
    > ensure that all tasks either freeze themselves before the error path in
    > try_to_freeze_tasks() is executed, or remain unfrozen.
    >
    > I'll try to prepare a patch to illustrate this, but right now I'm too tired to
    > do it. :-)

    Something like this, perhaps:

    ---
    include/linux/freezer.h | 10 +++-------
    kernel/power/process.c | 18 ++++++++++++++++--
    2 files changed, 19 insertions(+), 9 deletions(-)

    Index: linux-2.6.20-mm2/include/linux/freezer.h
    ===================================================================
    --- linux-2.6.20-mm2.orig/include/linux/freezer.h
    +++ linux-2.6.20-mm2/include/linux/freezer.h
    @@ -58,17 +58,13 @@ static inline void frozen_process(struct
    clear_tsk_thread_flag(p, TIF_FREEZE);
    }

    -extern void refrigerator(void);
    +extern int refrigerator(void);
    extern int freeze_processes(void);
    extern void thaw_processes(void);

    static inline int try_to_freeze(void)
    {
    - if (freezing(current)) {
    - refrigerator();
    - return 1;
    - } else
    - return 0;
    + return refrigerator();
    }

    /*
    @@ -104,7 +100,7 @@ static inline void freeze(struct task_st
    static inline int thaw_process(struct task_struct *p) { return 1; }
    static inline void frozen_process(struct task_struct *p) { BUG(); }

    -static inline void refrigerator(void) {}
    +static inline int refrigerator(void) { return 0; }
    static inline int freeze_processes(void) { BUG(); return 0; }
    static inline void thaw_processes(void) {}

    Index: linux-2.6.20-mm2/kernel/power/process.c
    ===================================================================
    --- linux-2.6.20-mm2.orig/kernel/power/process.c
    +++ linux-2.6.20-mm2/kernel/power/process.c
    @@ -24,6 +24,8 @@
    #define FREEZER_KERNEL_THREADS 0
    #define FREEZER_USER_SPACE 1

    +spinlock_t refrigerator_lock;
    +
    static inline int freezeable(struct task_struct * p)
    {
    if ((p == current) ||
    @@ -34,15 +36,23 @@ static inline int freezeable(struct task
    }

    /* Refrigerator is place where frozen processes are stored :-). */
    -void refrigerator(void)
    +int refrigerator(void)
    {
    /* Hmm, should we be allowed to suspend when there are realtime
    processes around? */
    long save;
    +
    + spin_lock(&refrigerator_lock);
    + if (freezing(current)) {
    + frozen_process(current);
    + spin_unlock(&refrigerator_lock);
    + } else {
    + spin_unlock(&refrigerator_lock);
    + return 0;
    + }
    save = current->state;
    pr_debug("%s entered refrigerator\n", current->comm);

    - frozen_process(current);
    spin_lock_irq(&current->sighand->siglock);
    recalc_sigpending(); /* We sent fake signal, clean it up */
    spin_unlock_irq(&current->sighand->siglock);
    @@ -53,6 +63,7 @@ void refrigerator(void)
    }
    pr_debug("%s left refrigerator\n", current->comm);
    current->state = save;
    + return 1;
    }

    static inline void freeze_process(struct task_struct *p)
    @@ -143,6 +154,7 @@ static unsigned int try_to_freeze_tasks(
    "kernel threads",
    TIMEOUT / HZ, todo);
    read_lock(&tasklist_lock);
    + spin_lock(&refrigerator_lock);
    do_each_thread(g, p) {
    if (is_user_space(p) == !freeze_user_space)
    continue;
    @@ -152,6 +164,7 @@ static unsigned int try_to_freeze_tasks(

    cancel_freezing(p);
    } while_each_thread(g, p);
    + spin_unlock(&refrigerator_lock);
    read_unlock(&tasklist_lock);
    }

    @@ -169,6 +182,7 @@ int freeze_processes(void)
    unsigned int nr_unfrozen;

    printk("Stopping tasks ... ");
    + spin_lock_init(&refrigerator_lock);
    nr_unfrozen = try_to_freeze_tasks(FREEZER_USER_SPACE);
    if (nr_unfrozen)
    return nr_unfrozen;
    -
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