Hi,
On Sunday 05 February 2006 10:38, Andrew Morton wrote:
> "Rafael J. Wysocki" <[email protected]> wrote:
> > This patch allows swsusp to freeze processes successfully under heavy load
> > by freezing userspace processes before kernel threads.
> >
> > ...
> >
> > /* 0 = success, else # of processes that we failed to stop */
> > int freeze_processes(void)
> > {
> > - int todo;
> > + int todo, nr_user, user_frozen;
> > unsigned long start_time;
> > struct task_struct *g, *p;
> > unsigned long flags;
> >
> > printk( "Stopping tasks: " );
> > start_time = jiffies;
> > + user_frozen = 0;
> > do {
> > - todo = 0;
> > + nr_user = todo = 0;
> > read_lock(&tasklist_lock);
> > do_each_thread(g, p) {
> > if (!freezeable(p))
> > continue;
> > if (frozen(p))
> > continue;
> > -
> > - freeze(p);
> > - spin_lock_irqsave(&p->sighand->siglock, flags);
> > - signal_wake_up(p, 0);
> > - spin_unlock_irqrestore(&p->sighand->siglock, flags);
> > - todo++;
> > + if (p->mm && !(p->flags & PF_BORROWED_MM)) {
> > + /* The task is a user-space one.
> > + * Freeze it unless there's a vfork completion
> > + * pending
> > + */
> > + if (!p->vfork_done)
> > + freeze_process(p);
> > + nr_user++;
> > + } else {
> > + /* Freeze only if the user space is frozen */
> > + if (user_frozen)
> > + freeze_process(p);
> > + todo++;
> > + }
> > } while_each_thread(g, p);
> > read_unlock(&tasklist_lock);
> > + todo += nr_user;
> > + if (!user_frozen && !nr_user) {
> > + sys_sync();
> > + start_time = jiffies;
> > + }
> > + user_frozen = !nr_user;
> > yield(); /* Yield is okay here */
> > - if (todo && time_after(jiffies, start_time + TIMEOUT)) {
> > - printk( "\n" );
> > - printk(KERN_ERR " stopping tasks timed out (%d tasks remaining)\n", todo );
> > + if (todo && time_after(jiffies, start_time + TIMEOUT))
> > break;
>
> The logic in that loop makes my brain burst.
>
> What happens if a process does vfork();sleep(100000000)?
The freezing of processes will fail due to the timeout.
Without the if (!p->vfork_done) it would fail too, because the child would
be frozen and the parent would wait for the vfork completion in the
TASK_UNINTERRUPTIBLE state (ie. unfreezeable). But in that case
we have a race between the "freezer" and the child process
(ie. if the child gets frozen before it completes the vfork completion, the
paret will be unfreezeable) which sometimes leads to a failure when it
should not. [We have a test case showing this.]
Greetings,
Rafael
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