Messages in this thread | | | Date | Tue, 20 Feb 2007 03:12:09 +0300 | From | Oleg Nesterov <> | Subject | Re: freezer problems |
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On 02/20, Rafael J. Wysocki wrote: > > On Monday, 19 February 2007 23:41, Oleg Nesterov wrote: > > On 02/19, Rafael J. Wysocki wrote: > > > > > > On Monday, 19 February 2007 21:23, Oleg Nesterov wrote: > > > > > > > > @@ -199,6 +189,10 @@ static void thaw_tasks(int thaw_user_spa > > > > > > > > > > do_each_thread(g, p) { > > > > > + if (freezer_should_skip(p)) > > > > > + cancel_freezing(p); > > > > > + } while_each_thread(g, p); > > > > > + do_each_thread(g, p) { > > > > > if (!freezeable(p)) > > > > > continue; > > > > > > > > Any reason for 2 separate do_each_thread() loops ? > > > > > > Yes. If there is a "freeze" request pending for the vfork parent (TIF_FREEZE > > > set), we have to cancel it before the child is unfrozen, since otherwise the > > > parent may go freezing after we try to reset PF_FROZEN for it. > > > > I see, thanks... thaw_process() doesn't take TIF_FREEZE into account. > > > > But doesn't this mean we have a race? > > > > Suppose that try_to_freeze_tasks() failed. It does cancel_freezing() for each > > process before return, but what if some thread already checked TIF_FREEZE and > > (for simplicity) it is preempted before frozen_process() in refrigerator(). > > > > thaw_tasks() runs, ignores this task (P), returns. P gets CPU, and becomes > > frozen, but nobody will thaw it. > > > > No? > > Well, I think this is highly theoretical. Namely, try_to_freeze_tasks() only > fails after the timeout that's currently set to 20 sec., and it yields the CPU > in each iteration of the main loop. The task in question would have to refuse > being frozen for 20 sec. and then suddenly decide to freeze itself right before > try_to_freeze_tasks() checks the timeout for the very last time. Then, it > would have to get preempted at this very moment and stay unfrozen at least > until thaw_tasks() starts running and in fact even longer.
Yes, yes, it is pure theroretical,
> I think we may avoid this by making try_to_freeze_tasks() sleep for some time > after it has reset TIF_FREEZE for all tasks in the error path, if anyone is > ever able to trigger it.
This makes this race (pure theroretical) ** 2 :)
Still. May be it make sense to introduce cancel_freezing_and_thaw() function (not right now) which stops the task from sleeping in refrigirator reliably. I didn't think much about this, but it looks like we can fix coredump/exec problems. Of course, this is not so important, we can ignore them at least for now (->vfork_done is different, should be imho solved, because any user can block freezer forever).
The fix:
refrigerator:
+ // we are going to call do_exit() really soon, + // we have a pending SIGKILL + if (current->signal->flags & SIGNAL_GROUP_EXIT) + return;
frozen_process(current); ...
zap_other_threads:
for_each_subthread() { ...
+ // ---- SIGNAL_GROUP_EXIT is set ------ + // we can check sig->group_exit_task to detect de_thread, + // but perhaps it doesn't hurt if the caller is do_group_exit + cancel_freezing_and_thaw(p); sigaddset(&t->pending.signal, SIGKILL); signal_wake_up(t, 1); }
This way execer reliably kills all sub-threads and proceeds without blocking try_to_freeze_tasks(). The same change could be done for zap_process() to fix coredump.
Oleg.
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