* KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> [2008-12-11 09:37:33]:
> thank you for this work.
>
> On Wed, 10 Dec 2008 15:36:55 -0800
> menage@xxxxxxxxxx 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@xxxxxxxxxx>
> >
> > ---
> >
> > 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
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