lkml.org 
[lkml]   [2011]   [Aug]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
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.



\
 
 \ /
  Last update: 2011-08-25 17:31    [from the cache]
©2003-2011 Jasper Spaans