Commit 696ac172 authored by Johannes Weiner's avatar Johannes Weiner Committed by Linus Torvalds

mm: memcg: fix test for child groups

When memcg code needs to know whether any given memcg has children, it
uses the cgroup child iteration primitives and returns true/false
depending on whether the iteration loop is executed at least once or
not.

Because a cgroup's list of children is RCU protected, these primitives
require the RCU read-lock to be held, which is not the case for all
memcg callers.  This results in the following splat when e.g.  enabling
hierarchy mode:

  WARNING: CPU: 3 PID: 1 at kernel/cgroup.c:3043 css_next_child+0xa3/0x160()
  CPU: 3 PID: 1 Comm: systemd Not tainted 3.12.0-rc5-00117-g83f11a9c-dirty #18
  Hardware name: LENOVO 3680B56/3680B56, BIOS 6QET69WW (1.39 ) 04/26/2012
  Call Trace:
    dump_stack+0x54/0x74
    warn_slowpath_common+0x78/0xa0
    warn_slowpath_null+0x1a/0x20
    css_next_child+0xa3/0x160
    mem_cgroup_hierarchy_write+0x5b/0xa0
    cgroup_file_write+0x108/0x2a0
    vfs_write+0xbd/0x1e0
    SyS_write+0x4c/0xa0
    system_call_fastpath+0x16/0x1b

In the memcg case, we only care about children when we are attempting to
modify inheritable attributes interactively.  Racing with deletion could
mean a spurious -EBUSY, no problem.  Racing with addition is handled
just fine as well through the memcg_create_mutex: if the child group is
not on the list after the mutex is acquired, it won't be initialized
from the parent's attributes until after the unlock.
Signed-off-by: default avatarJohannes Weiner <hannes@cmpxchg.org>
Acked-by: default avatarMichal Hocko <mhocko@suse.cz>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 0056f4e6
......@@ -4959,31 +4959,18 @@ static void mem_cgroup_reparent_charges(struct mem_cgroup *memcg)
} while (usage > 0);
}
/*
* This mainly exists for tests during the setting of set of use_hierarchy.
* Since this is the very setting we are changing, the current hierarchy value
* is meaningless
*/
static inline bool __memcg_has_children(struct mem_cgroup *memcg)
{
struct cgroup_subsys_state *pos;
/* bounce at first found */
css_for_each_child(pos, &memcg->css)
return true;
return false;
}
/*
* Must be called with memcg_create_mutex held, unless the cgroup is guaranteed
* to be already dead (as in mem_cgroup_force_empty, for instance). This is
* from mem_cgroup_count_children(), in the sense that we don't really care how
* many children we have; we only need to know if we have any. It also counts
* any memcg without hierarchy as infertile.
*/
static inline bool memcg_has_children(struct mem_cgroup *memcg)
{
return memcg->use_hierarchy && __memcg_has_children(memcg);
lockdep_assert_held(&memcg_create_mutex);
/*
* The lock does not prevent addition or deletion to the list
* of children, but it prevents a new child from being
* initialized based on this parent in css_online(), so it's
* enough to decide whether hierarchically inherited
* attributes can still be changed or not.
*/
return memcg->use_hierarchy &&
!list_empty(&memcg->css.cgroup->children);
}
/*
......@@ -5063,7 +5050,7 @@ static int mem_cgroup_hierarchy_write(struct cgroup_subsys_state *css,
*/
if ((!parent_memcg || !parent_memcg->use_hierarchy) &&
(val == 1 || val == 0)) {
if (!__memcg_has_children(memcg))
if (list_empty(&memcg->css.cgroup->children))
memcg->use_hierarchy = val;
else
retval = -EBUSY;
......
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