Re: [RFC][PATCH -mm 1/5] PM: Make freeze_processes SMP-safe

Rafael J. Wysocki
2006-11-26T23:09:55Z

Hi,

On Sunday, 26 November 2006 20:48, Pavel Machek wrote:
> Hi!
>
> > Index: linux-2.6.19-rc6-mm1/kernel/power/process.c
> > ===================================================================
> > --- linux-2.6.19-rc6-mm1.orig/kernel/power/process.c 2006-11-25 21:26:52.000000000 +0100
> > +++ linux-2.6.19-rc6-mm1/kernel/power/process.c 2006-11-26 14:17:11.000000000 +0100
> > @@ -28,8 +28,7 @@ static inline int freezeable(struct task
> > if ((p == current) ||
> > (p->flags & PF_NOFREEZE) ||
> > (p->exit_state == EXIT_ZOMBIE) ||
> > - (p->exit_state == EXIT_DEAD) ||
> > - (p->state == TASK_STOPPED))
> > + (p->exit_state == EXIT_DEAD))
> > return 0;
> > return 1;
> > }
> > @@ -61,10 +60,13 @@ static inline void freeze_process(struct
> > unsigned long flags;
> >
> > if (!freezing(p)) {
> > - freeze(p);
> > - spin_lock_irqsave(&p->sighand->siglock, flags);
> > - signal_wake_up(p, 0);
> > - spin_unlock_irqrestore(&p->sighand->siglock, flags);
> > + rmb();
>
> If frozen is atomic_t, do we need memory barrier?

I think so. For example on x86-64 atomic_read() is just a read.

> > + if (!frozen(p)) {
> > + freeze(p);
> > + spin_lock_irqsave(&p->sighand->siglock, flags);
> > + signal_wake_up(p, 0);
> > + spin_unlock_irqrestore(&p->sighand->siglock, flags);
> > + }
> > }
> > }
> >
> > @@ -90,11 +92,12 @@ static unsigned int try_to_freeze_tasks(
> > {
> > struct task_struct *g, *p;
> > unsigned long end_time;
> > - unsigned int todo;
> > + unsigned int todo, nr_stopped;
> >
> > end_time = jiffies + TIMEOUT;
> > do {
> > todo = 0;
> > + nr_stopped = 0;
> > read_lock(&tasklist_lock);
> > do_each_thread(g, p) {
> > if (!freezeable(p))
> > @@ -103,6 +106,10 @@ static unsigned int try_to_freeze_tasks(
> > if (frozen(p))
> > continue;
> >
> > + if (p->state == TASK_STOPPED) {
> > + nr_stopped++;
> > + continue;
> > + }
> > if (p->state == TASK_TRACED &&
> > (frozen(p->parent) ||
> > p->parent->state == TASK_STOPPED)) {
> > @@ -128,6 +135,21 @@ static unsigned int try_to_freeze_tasks(
> > } while_each_thread(g, p);
> > read_unlock(&tasklist_lock);
> > yield(); /* Yield is okay here */
> > + if (!todo) {
> > + /* Make sure that none of the stopped processes has
> > + * received the continuation signal after we checked
> > + * last time.
> > + */
>
> I do not like the counting idea; it should be simpler to just check if
> all the processes are still stopped.

I thought about that but didn't invent anything reasonable enough.

> But I'm not sure if this is enough. What if signal is being delivered
> on another CPU while freezing, still being delivered while this second
> check runs, and then SIGCONT is delivered?

Hm, is this possible in practice? I mean, if todo is 0 and nr_stopped doesn't
change, then there are no processes that can send the SIGCONT (unless someone
creates a kernel thread with PF_NOFREEZE that will do just that).

Anyway, for now I've no idea how to fix this properly. Will think about it
tomorrow.

Greetings,
Rafael

--
You never change things by fighting the existing reality.
R. Buckminster Fuller