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