Re: [RFC][PATCH 1/3] CGroups: Add a per-subsystem hierarchy_mutex

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2008-12-11 09:37:33]:

> thank you for this work.
> 
> On Wed, 10 Dec 2008 15:36:55 -0800
> menage@google.com wrote:
> 
> > This patch adds a hierarchy_mutex to the cgroup_subsys object that
> > protects changes to the hierarchy observed by that subsystem. It is
> > taken by the cgroup subsystem (in addition to cgroup_mutex) for the
> > following operations:
> > 
> > - linking a cgroup into that subsystem's cgroup tree
> > - unlinking a cgroup from that subsystem's cgroup tree
> > - moving the subsystem to/from a hierarchy (including across the
> >   bind() callback)
> > 
> o.k. usage is very clear.
> 
> > Thus if the subsystem holds its own hierarchy_mutex, it can safely
> > traverse its own hierarchy.
> > 
> > Signed-off-by: Paul Menage <menage@google.com>
> > 
> > ---
> > 
> >  Documentation/cgroups/cgroups.txt |    2 +-
> >  include/linux/cgroup.h            |    9 ++++++++-
> >  kernel/cgroup.c                   |   37 +++++++++++++++++++++++++++++++++++--
> >  3 files changed, 44 insertions(+), 4 deletions(-)
> > 
> > Index: hierarchy_lock-mmotm-2008-12-09/include/linux/cgroup.h
> > ===================================================================
> > --- hierarchy_lock-mmotm-2008-12-09.orig/include/linux/cgroup.h
> > +++ hierarchy_lock-mmotm-2008-12-09/include/linux/cgroup.h
> > @@ -337,8 +337,15 @@ struct cgroup_subsys {
> >  #define MAX_CGROUP_TYPE_NAMELEN 32
> >  	const char *name;
> >  
> > -	struct cgroupfs_root *root;
> > +	/*
> > +	 * Protects sibling/children links of cgroups in this
> > +	 * hierarchy, plus protects which hierarchy (or none) the
> > +	 * subsystem is a part of (i.e. root/sibling)
> > +	 */
> > +	struct mutex hierarchy_mutex;
> >  
> > +	/* Protected by this->hierarchy_mutex and cgroup_lock() */
> > +	struct cgroupfs_root *root;
> >  	struct list_head sibling;
> >  };
> >  
> > Index: hierarchy_lock-mmotm-2008-12-09/kernel/cgroup.c
> > ===================================================================
> > --- hierarchy_lock-mmotm-2008-12-09.orig/kernel/cgroup.c
> > +++ hierarchy_lock-mmotm-2008-12-09/kernel/cgroup.c
> > @@ -714,23 +714,26 @@ static int rebind_subsystems(struct cgro
> >  			BUG_ON(cgrp->subsys[i]);
> >  			BUG_ON(!dummytop->subsys[i]);
> >  			BUG_ON(dummytop->subsys[i]->cgroup != dummytop);
> > +			mutex_lock(&ss->hierarchy_mutex);
> >  			cgrp->subsys[i] = dummytop->subsys[i];
> >  			cgrp->subsys[i]->cgroup = cgrp;
> >  			list_move(&ss->sibling, &root->subsys_list);
> >  			ss->root = root;
> >  			if (ss->bind)
> >  				ss->bind(ss, cgrp);
> > -
> > +			mutex_unlock(&ss->hierarchy_mutex);
> >  		} else if (bit & removed_bits) {
> >  			/* We're removing this subsystem */
> >  			BUG_ON(cgrp->subsys[i] != dummytop->subsys[i]);
> >  			BUG_ON(cgrp->subsys[i]->cgroup != cgrp);
> > +			mutex_lock(&ss->hierarchy_mutex);
> >  			if (ss->bind)
> >  				ss->bind(ss, dummytop);
> >  			dummytop->subsys[i]->cgroup = dummytop;
> >  			cgrp->subsys[i] = NULL;
> >  			subsys[i]->root = &rootnode;
> >  			list_move(&ss->sibling, &rootnode.subsys_list);
> > +			mutex_unlock(&ss->hierarchy_mutex);
> >  		} else if (bit & final_bits) {
> >  			/* Subsystem state should already exist */
> >  			BUG_ON(!cgrp->subsys[i]);
> > @@ -2326,6 +2329,29 @@ static void init_cgroup_css(struct cgrou
> >  	cgrp->subsys[ss->subsys_id] = css;
> >  }
> >  
> > +static void cgroup_lock_hierarchy(struct cgroupfs_root *root)
> > +{
> > +	/* We need to take each hierarchy_mutex in a consistent order */
> > +	int i;
> > +
> > +	for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
> > +		struct cgroup_subsys *ss = subsys[i];
> > +		if (ss->root == root)
> > +			mutex_lock_nested(&ss->hierarchy_mutex, i);
> > +	}
> > +}
> > +
> > +static void cgroup_unlock_hierarchy(struct cgroupfs_root *root)
> > +{
> > +	int i;
> > +
> > +	for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
> > +		struct cgroup_subsys *ss = subsys[i];
> > +		if (ss->root == root)
> > +			mutex_unlock(&ss->hierarchy_mutex);
> > +	}
> > +}
> > +
> Maybe no problem..but I don't like releasing lock in the order of acquiring lock.
> 
> 	for (i = CGROUP_SUBSYS_COUNT - 1; i >=0; i--)  ?
>

Good point!

-- 
	Balbir