[Devel] Re: [PATCH v7 1/3] cgroups: read-write lock CLONE_THREAD forking per threadgroup

[Ben Blum]

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 at andrew.cmu.edu> wrote:
> 
> > Adds functionality to read/write lock CLONE_THREAD fork()ing per-threadgroup
> > 
> > From: Ben Blum <bblum at 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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