[PATCH v4 1/2] cgroups: read-write lock CLONE_THREAD forking per threadgroup

[Ben Blum]

On Tue, Aug 03, 2010 at 08:44:01PM -0700, Paul Menage wrote:
>  On Fri, Jul 30, 2010 at 4:57 PM, Ben Blum <bblum at andrew.cmu.edu> wrote:
> > +	 * The threadgroup_fork_lock prevents threads from forking with
> > +	 * CLONE_THREAD while held for writing. Use this for fork-sensitive
> > +	 * threadgroup-wide operations. It's taken for reading in fork.c in
> > +	 * copy_process().
> > +	 * Currently only needed write-side by cgroups.
> > +	 */
> > +	struct rw_semaphore threadgroup_fork_lock;
> > +#endif
> 
> I'm not sure how best to word this comment, but I'd prefer something like:
> 
> "The threadgroup_fork_lock is taken in read mode during a CLONE_THREAD
> fork operation; taking it in write mode prevents the owning
> threadgroup from adding any new threads and thus allows you to
> synchronize against the addition of unseen threads when performing
> threadgroup-wide operations. New-process forks (without CLONE_THREAD)
> are not affected."

That sounds good.

> As far as the #ifdef mess goes, it's true that some people don't have
> CONFIG_CGROUPS defined. I'd imagine that these are likely to be
> embedded systems with a fairly small number of processes and threads
> per process. Are there really any such platforms where the cost of a
> single extra rwsem per process is going to make a difference either in
> terms of memory or lock contention? I think you should consider making
> these additions unconditional.

That's certainly an option, but I think it would be clean enough to put
static inline functions just under the signal_struct definition.
Thoughts?

> 
> Paul
> 

-- Ben