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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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