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    SubjectRe: [RFC PATCH(Experimental) 0/4] Freezer based Cpu-hotplug
    Date
    On Sunday, 18 February 2007 17:11, Oleg Nesterov wrote:
    > On 02/18, Rafael J. Wysocki wrote:
    > >
    > > > On Sunday, 18 February 2007 12:31, Oleg Nesterov wrote:
    > > > >
    > > > > A very vague idea: what if parent will do
    > > > >
    > > > > current->flags |= PF_PLEASE_CONSIDER_ME_AS_FROZEN_BUT_SET_TIF_FREEZE
    > > > > wait_for_completion(&vfork);
    > > > > try_to_freeze();
    > > > >
    > > > > ?
    > >
    > > Hm, what about the following patch instead?
    > >
    > > The problem is that if the child enters the refrigeratior, we can't freeze the
    > > parent, because it's uninterruptible, but the child knows the parent will be
    > > uninterruptible until it exits, so the child can mark the parent as frozen.
    > >
    > > --- linux-2.6.20-mm2.orig/kernel/power/process.c 2007-02-18 15:43:30.000000000 +0100
    > > +++ linux-2.6.20-mm2/kernel/power/process.c 2007-02-18 16:09:53.000000000 +0100
    > > @@ -39,6 +39,13 @@ void refrigerator(void)
    > > /* Hmm, should we be allowed to suspend when there are realtime
    > > processes around? */
    > > long save;
    > > +
    > > + /* The parent is uninterruptible and will stay so until this task exits,
    > > + * so we can mark it as frozen.
    > > + */
    > > + if (current->vfork_done)
    > > + frozen_process(current->parent);
    >
    > This is not safe. task->flags is not atomic, we can change ->flags only
    > if we know the task won't touch it itself (ptrace, thaw_process).
    > The parent could be interrupted, irq may play with current->flags (slab,
    > for example).
    >
    > Please note that ->parent may do things like ptrace_notify() before
    > it actually sleeps on ->vfork_done. This means that even if we could
    > set PF_FROZEN in a safe manner, this doesn't look like a good idea.
    >
    > > +
    > > + if (current->vfork_done && frozen(current->parent))
    > > + current->parent->flags &= ~PF_FROZEN;
    > > }
    >
    > Why? If the code above works, we shouldn't take care about frozen
    > ->parent?

    I've added this for symmetry. thaw_tasks() should reset PF_FROZEN for it
    anyway.

    Okay, so I'll post the patch that implements your idea in the other thread.

    Greetings,
    Rafael
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