Re: [RFC PATCH 1/2] Fix PF_NOFREEZE and freezeable race

Oleg Nesterov
2007-04-20T18:02:51Z

On 04/19, Rafael J. Wysocki wrote:
>
> On Thursday, 19 April 2007 14:02, Gautham R Shenoy wrote:
> > This patch fixes the race pointed out by Oleg Nesterov.
> >
> > * Freezer marks a thread as freezeable.
> > * The thread now marks itself PF_NOFREEZE causing it to
> > freeze on calling try_to_freeze(). Thus the task is frozen, even though
> > it doesn't want to.
> > * Subsequent thaw_processes() will also fail to thaw the task since it is
> > marked PF_NOFREEZE.
> >
> > Avoid this problem by checking the current task's PF_NOFREEZE status in the
> > refrigerator before marking current as frozen.
> >
> > Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>
>
> Looks good, although I'm not sure if we don't need to call recalc_sigpending()
> for tasks that turn out to be PF_NOFREEZE.

I agree, we should clear TIF_SIGPENDING. It is not so critical for user-space
tasks, but for the kernel thread it may remain pending forever, causing subtle
failures.

Gautham, isn't it possible to make a more simpler patch ? Just add PF_NOFREEZE
check to frozen_process,

static inline void frozen_process(struct task_struct *p)
{
if (!unlikely(current->flags & PF_NOFREEZE)) {
p->flags |= PF_FROZEN;
wmb();
}
clear_tsk_thread_flag(p, TIF_FREEZE);
}

No?

Oleg.