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


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