Messages in this thread | | | Date | Sat, 14 Apr 2007 23:34:06 +0400 | From | Oleg Nesterov <> | Subject | Re: [PATCH] kthread: Enhance kthread_stop to abort interruptible sleeps |
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On 04/14, Eric W. Biederman wrote: > > Oleg Nesterov <oleg@tv-sign.ru> writes: > > > On 04/13, Eric W. Biederman wrote: > >> > >> +static inline int __kthread_should_stop(struct task_struct *tsk) > >> +{ > >> + return test_tsk_thread_flag(tsk, TIF_KTHREAD_STOP); > >> +} > > > > Am I blind? Where does copy_process/dup_task_struct clears unwanted > > flags in thread_info->flags ? > > Good question. It is only a real problem if someone forks a kernel > thread after we ask it to die but, it does appear to be an issue. > With this usage and the same usage by the process freezer. > > We do have these lines in copy_process... > > clear_tsk_thread_flag(p, TIF_SIGPENDING); > init_sigpending(&p->pending); > > I don't know what we want to do about TIF_KTHREAD_STOP and TIF_FREEZE.
Perhaps we need _TIF_CLEAR_ON_FORK_MASK. Probably doesn't matter right now, but still it is not imho safe in general.
> Right now we will go allow our merry way until we hit: > > recalc_sigpending(); > if (signal_pending(current)) { > spin_unlock(¤t->sighand->siglock); > write_unlock_irq(&tasklist_lock); > retval = -ERESTARTNOINTR; > goto bad_fork_cleanup_namespaces; > } > > And copy_process will fail. Since that is an expected failure point > that actually seems like reasonable behavior in this case if you > are being frozen or are being told to die you can't fork. > > It does ensure that these additional kernel flags won't make it > onto new instances of struct task_struct. Which is the important > thing from a correctness standpoint.
Note that we set TIF_FREEZE and TIF_KTHREAD_STOP outside of ->siglock, so both flags can leak onto the child. Again, not a problem right now. TIF_KTHREAD_STOP doesn't matter unless process was created vi kthread_create(), but in that case it can't inherit TIF_KTHREAD_STOP.
Oleg.
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