Messages in this thread | | | Date | Mon, 10 Oct 2011 10:11:05 -0700 | From | Tejun Heo <> | Subject | Re: [PATCH 3/4] threadgroup: extend threadgroup_lock() to cover exit and exec |
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Hello, Oleg.
Sorry about the very long delay. I moved cross atlantic and had a pretty long vacation while doing it. Hope you can still remember some of this one. :)
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?
> > + /* > > + * 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.
Also, it makes the mechanism unnecessarily cgroup-specific without gaining much if anything. It's per-threadgroup rwsem so contention isn't a problem and narrowing critical section isn't likely to be beneficial (maybe slightly increase the chance of the cacheline for the lock to be hot?).
Thank you.
-- tejun
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