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    SubjectRe: [RFC][PATCH 2/3] CGroups: Use hierarchy_mutex in memory controller
    On Wed, 10 Dec 2008 15:36:56 -0800
    menage@google.com wrote:

    > This patch updates the memory controller to use its hierarchy_mutex
    > rather than calling cgroup_lock() to protected against
    > cgroup_mkdir()/cgroup_rmdir() from occurring in its hierarchy.
    >
    > Signed-off-by: Paul Menage <menage@google.com>
    >
    Hmm, can we avoid following kind of dead-lock by this ?

    Assume subsys A and B is on the same hierarchy, A's subsys ID is smaller than B's.
    (I assume memory and cpuset for A and B, but just an example.)

    an operation like rmdir() in somewhere.
    hierarchy_lock for A (acquired)
    hierarchy_lock for B (waiting)

    in subsys A.
    mmap_sem (acquired)
    hierarchy_lock for A (waiting)
    in subsys B.
    hierarchy_lock for B (aquired)
    mmap_sem (waiting)



    Thanks,
    -Kame

    > ---
    > mm/memcontrol.c | 14 ++++++--------
    > 1 file changed, 6 insertions(+), 8 deletions(-)
    >
    > Index: hierarchy_lock-mmotm-2008-12-09/mm/memcontrol.c
    > ===================================================================
    > --- hierarchy_lock-mmotm-2008-12-09.orig/mm/memcontrol.c
    > +++ hierarchy_lock-mmotm-2008-12-09/mm/memcontrol.c
    > @@ -154,7 +154,7 @@ struct mem_cgroup {
    >
    > /*
    > * While reclaiming in a hiearchy, we cache the last child we
    > - * reclaimed from. Protected by cgroup_lock()
    > + * reclaimed from. Protected by hierarchy_mutex
    > */
    > struct mem_cgroup *last_scanned_child;
    > /*
    > @@ -529,7 +529,7 @@ unsigned long mem_cgroup_isolate_pages(u
    >
    > /*
    > * This routine finds the DFS walk successor. This routine should be
    > - * called with cgroup_mutex held
    > + * called with hierarchy_mutex held
    > */
    > static struct mem_cgroup *
    > mem_cgroup_get_next_node(struct mem_cgroup *curr, struct mem_cgroup *root_mem)
    > @@ -598,7 +598,7 @@ mem_cgroup_get_first_node(struct mem_cgr
    > /*
    > * Scan all children under the mem_cgroup mem
    > */
    > - cgroup_lock();
    > + mutex_lock(&mem_cgroup_subsys.hierarchy_mutex);
    > if (list_empty(&root_mem->css.cgroup->children)) {
    > ret = root_mem;
    > goto done;
    > @@ -619,7 +619,7 @@ mem_cgroup_get_first_node(struct mem_cgr
    >
    > done:
    > root_mem->last_scanned_child = ret;
    > - cgroup_unlock();
    > + mutex_unlock(&mem_cgroup_subsys.hierarchy_mutex);
    > return ret;
    > }
    >
    > @@ -683,18 +683,16 @@ static int mem_cgroup_hierarchical_recla
    > while (next_mem != root_mem) {
    > if (next_mem->obsolete) {
    > mem_cgroup_put(next_mem);
    > - cgroup_lock();
    > next_mem = mem_cgroup_get_first_node(root_mem);
    > - cgroup_unlock();
    > continue;
    > }
    > ret = try_to_free_mem_cgroup_pages(next_mem, gfp_mask, noswap,
    > get_swappiness(next_mem));
    > if (mem_cgroup_check_under_limit(root_mem))
    > return 0;
    > - cgroup_lock();
    > + mutex_lock(&mem_cgroup_subsys.hierarchy_mutex);
    > next_mem = mem_cgroup_get_next_node(next_mem, root_mem);
    > - cgroup_unlock();
    > + mutex_unlock(&mem_cgroup_subsys.hierarchy_mutex);
    > }
    > return ret;
    > }
    >
    > --
    >



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