Re: [PATCH UPDATED 03/10] threadgroup: extend threadgroup_lock() tocover exit and exec

[Linus Torvalds]

On Thu, Nov 24, 2011 at 2:50 PM, Tejun Heo <tj@xxxxxxxxxx> wrote:
>
> Linus, this is something being scheduled for the next merge window.
> It extends threadgroup locking which cgroup used to do to exclude only
> fork path so that it includes exit and exec paths.  threadgroup
> locking is used by cgroup to implement process-scope cgroup migration.
>
> Migration happens in multiple steps, at each of which the matching
> method of each cgroup plugin is called.  When a whole process is being
> migrated, methods being called at those different steps expect to see
> consistent image of the thread group.

Ugh.

I see why you want to do this, but I really don't much like some of it.

I've got a few questions:

 - do you even *need* two separate locks at all? If you extend the
threadgroup_fork_lock to cover exec/exit too (and rename it), then why
does that separate cred_guard_mutex make sense at all?

  Taking two locks for the common exit case seems disgusting. What do
the separate locks buy us?

 - could we possible avoid the lock(s) entirely for the
single-threaded case? The fact that ptrace wants to serialize makes me
say "maybe we can't avoid it", but I thought I'd ask. Even if we do
need/want two separate locks, do we really want to take them both for
the case that shouldn't really need even a single one?

Hmm? Simplifying the locking rules is always a good idea, and I think
this seems to make some of it more straightforward, but at the same
time I shudder when I look at some of the patches in the series that
nest locking three locks deep. Ugh.

                 Linus
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