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    SubjectRe: [PATCH v7 1/3] cgroups: read-write lock CLONE_THREAD forking per threadgroup
    On Fri, Feb 04, 2011 at 01:36:57PM -0800, Andrew Morton wrote:
    > On Fri, 4 Feb 2011 16:25:15 -0500
    > Ben Blum <bblum@andrew.cmu.edu> wrote:
    >
    > > 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.
    >
    > What about adding a new header file which includes rwsem.h and sched.h
    > and then defines the new interfaces?

    Er, I mean the definition of signal_struct needs rwsem.h as well, not
    just the threadgroup_fork_* functions. (And I suspect moving
    signal_struct somewhere else would give bigger problems...)


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