On Wed, Feb 21, 2007 at 07:13:40PM +0100, Rafael J. Wysocki wrote:
> 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):
Ah, OK, that should work much better from a lock-contention viewpoint!
Thanx, Paul
> ---
> 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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