Re: freezer problems

[Rafael J. Wysocki]

On Wednesday, 21 February 2007 19:14, Paul E. McKenney wrote:
> 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!

Yes.  Here's the current version (try_to_freeze() is unchanged, so the lock
is only taken by the tasks that are going to freeze, or so they think):

---
 kernel/power/process.c |   15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

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
 
+static spinlock_t refrigerator_lock;
+
 static inline int freezeable(struct task_struct * p)
 {
 	if ((p == current) ||
@@ -39,10 +41,18 @@ void 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;
+	}
 	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);
@@ -143,6 +153,7 @@ 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 +163,7 @@ 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 +181,7 @@ 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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