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    SubjectRe: [PATCH 06/16] freezer: make exiting tasks properly unfreezable
    On 08/24, Matt Helsley wrote:
    >
    > On Fri, Aug 19, 2011 at 04:16:12PM +0200, Tejun Heo wrote:
    > > There's no point in freezing an exiting task. The current code
    > > seemingly tries that by calling clear_freeze_flag() from exit_mm() but
    > > it's racy as freeze might happen afterwards.
    > >
    > > This patch removes the racy clear_freeze_flag() makes do_exit() set
    > > PF_NOFREEZE after PTRACE_EVENT_EXIT, after which freezing doesn't make
    > > sense.
    > >
    > > Signed-off-by: Tejun Heo <tj@kernel.org>
    > > ---
    > > kernel/exit.c | 8 ++++++--
    > > kernel/power/process.c | 3 +--
    > > 2 files changed, 7 insertions(+), 4 deletions(-)
    > >
    > > diff --git a/kernel/exit.c b/kernel/exit.c
    > > index 2913b35..ac58259 100644
    > > --- a/kernel/exit.c
    > > +++ b/kernel/exit.c
    > > @@ -679,8 +679,6 @@ static void exit_mm(struct task_struct * tsk)
    > > tsk->mm = NULL;
    > > up_read(&mm->mmap_sem);
    > > enter_lazy_tlb(mm, current);
    > > - /* We don't want this task to be frozen prematurely */
    > > - clear_freeze_flag(tsk);
    >
    > This patch doesn't look quite right. Perhaps that's because I'm
    > unclear why any of the freezer flags matter in the do_exit() path. How
    > can the task enter the refrigerator in do_exit()?

    it can't.

    > If we can't enter the refrigerator then why fiddle with these flags
    > here at all? That might explain why this race never introduced
    > problems.

    One problem is, try_to_freeze_tasks() shouldn't fail if we have a zombie.

    > Couldn't TIF_FREEZE already be set?

    Yes,

    > Setting PF_NOFREEZE only ensures
    > the flag won't get set "after" this point. To fix this I think we'd need
    > something more like:
    >
    > current->flags |= PF_NOFREEZE;
    > smp_wmb();
    > clear_freeze_flag(tsk);

    We don't really need this, we are not going to freeze.




    However. I thought about this too. In the long term I think we do need
    some cleanups to avoid the wrong TIF_SIGPENDING... but currently this
    is off-topic.

    Oleg.



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