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(¤t->sighand->siglock);
recalc_sigpending(); /* We sent fake signal, clean it up */
spin_unlock_irq(¤t->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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