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

[Paul Menage <menage-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>]

 On Fri, Jul 30, 2010 at 4:57 PM, Ben Blum <bblum-OM76b2Iv3yLQjUSlxSEPGw@public.gmane.org> 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."

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.

Paul