Messages in this thread | | | Date | Wed, 12 Oct 2011 19:51:04 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 3/4] threadgroup: extend threadgroup_lock() to cover exit and exec |
| |
Hi,
On 10/10, Tejun Heo wrote: > > Hope you can still remember some > of this one. :)
I am not sure ;)
> On Sun, Sep 18, 2011 at 07:37:23PM +0200, Oleg Nesterov wrote: > > > With this change, threadgroup_lock() guarantees that the target > > > threadgroup will remain stable - no new task will be added, no new > > > PF_EXITING will be set and exec won't happen. > > > > To me, this is the only "contradictory" change, > > What do you mean "contradictory"? Can you please elaborate?
Because, iirc, with this patch do_exit() does (almost) everything under rw_sem. OK, down_read() should be cheap, but still.
See also below.
> > > + /* > > > + * Release threadgroup and make sure we are holding no locks. > > > + */ > > > + threadgroup_change_done(tsk); > > > > I am wondering, can't we narrow the scope of threadgroup_change_begin/done > > in do_exit() path? > > > > The code after 4/4 still has to check PF_EXITING, this is correct. And yes, > > with this patch PF_EXITING becomes stable under ->group_rwsem. But, it seems, > > we do not really need this? > > > > I mean, can't we change cgroup_exit() to do threadgroup_change_begin/done > > instead? We do not really care about PF_EXITING, we only need to ensure that > > we can't race with cgroup_exit(), right? > > If we confine our usage to cgroup, excluding just against > cgroup_exit() might work although this is still a bit nasty. ie. some > callbacks might not expect half torn-down tasks in methods other than > the exit callback.
Oh, sorry, I don't understand... I already forgot the details.
> Also, it makes the mechanism unnecessarily cgroup-specific without > gaining much if anything.
Yes! And _personally_ I think it should be cgroup-specific, that is why I dislike the very fact do_exit() uses it directly. To me it would be cleaner to shift it into cgroup hooks. Yes, sure, this is subjective.
In fact I still hope we can kill this sem altogether, but so far I have no idea how we can do this. We do need the new per-process lock to protect (in particular) ->thread_group. It is quite possible that it should be rw_semaphore. But in this case we down_write(), not _read in exit/fork paths, and its scope should be small.
I do not think the current lock should have more users. Of course I can be wrong. And what exactly it protects? I mean copy_process(). Almost everything, but this simply connects to cgroup fork hooks.
Just my opinion, I am not going to insist.
Oleg.
| |