Re: freezer problems

[Paul E. McKenney]

On Tue, Feb 20, 2007 at 07:29:01PM +0100, Rafael J. Wysocki wrote:
> On Tuesday, 20 February 2007 01:32, Rafael J. Wysocki wrote:
> > On Tuesday, 20 February 2007 01:12, Oleg Nesterov wrote:
> > 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);

I hope we can do this without a global lock that is acquired on each
try_to_freeze() call!

> +	if (freezing(current)) {

Would it be possible to acquire the lock here instead, then recheck here?
Or use a per-thread lock?  (Yes, this would make the error checking path
have to acquire a very large number of threads, but...

							Thanx, Paul

> +		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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