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    SubjectRe: [PATCH v7 1/3] cgroups: read-write lock CLONE_THREAD forking per threadgroup
    On Mon, Jan 24, 2011 at 01:05:29PM -0800, Andrew Morton wrote:
    > On Sun, 26 Dec 2010 07:09:51 -0500
    > Ben Blum <bblum@andrew.cmu.edu> wrote:
    >
    > > Adds functionality to read/write lock CLONE_THREAD fork()ing per-threadgroup
    > >
    > > From: Ben Blum <bblum@andrew.cmu.edu>
    > >
    > > This patch adds an rwsem that lives in a threadgroup's signal_struct that's
    > > taken for reading in the fork path, under CONFIG_CGROUPS. If another part of
    > > the kernel later wants to use such a locking mechanism, the CONFIG_CGROUPS
    > > ifdefs should be changed to a higher-up flag that CGROUPS and the other system
    > > would both depend on.
    > >
    > > This is a pre-patch for cgroup-procs-write.patch.
    > >
    > > ...
    > >
    > > +/* See the declaration of threadgroup_fork_lock in signal_struct. */
    > > +#ifdef CONFIG_CGROUPS
    > > +static inline void threadgroup_fork_read_lock(struct task_struct *tsk)
    > > +{
    > > + down_read(&tsk->signal->threadgroup_fork_lock);
    > > +}
    > > +static inline void threadgroup_fork_read_unlock(struct task_struct *tsk)
    > > +{
    > > + up_read(&tsk->signal->threadgroup_fork_lock);
    > > +}
    > > +static inline void threadgroup_fork_write_lock(struct task_struct *tsk)
    > > +{
    > > + down_write(&tsk->signal->threadgroup_fork_lock);
    > > +}
    > > +static inline void threadgroup_fork_write_unlock(struct task_struct *tsk)
    > > +{
    > > + up_write(&tsk->signal->threadgroup_fork_lock);
    > > +}
    > > +#else
    >
    > Risky. sched.h doesn't include rwsem.h.
    >
    > We could make it do so, but almost every compilation unit in the kernel
    > includes sched.h. It would be nicer to make the kernel build
    > finer-grained, rather than blunter-grained. Don't be afraid to add new
    > header files if that is one way of doing this!

    Hmm, good point. But there's also:

    +#ifdef CONFIG_CGROUPS
    + struct rw_semaphore threadgroup_fork_lock;
    +#endif

    in the signal_struct, also in sched.h, which needs to be there. Or I
    could change it to a struct pointer with a forward incomplete
    declaration above, and kmalloc/kfree it? I don't like adding more
    alloc/free calls but don't know if it's more or less important than
    header granularity.

    -- Ben


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