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    Subject[PATCH 4.6 56/81] memcg: add RCU locking around css_for_each_descendant_pre() in memcg_offline_kmem()
    Date
    4.6-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Tejun Heo <tj@kernel.org>

    commit 3a06bb78ceeceacc86a1e31133a7944013f9775b upstream.

    memcg_offline_kmem() may be called from memcg_free_kmem() after a css
    init failure. memcg_free_kmem() is a ->css_free callback which is
    called without cgroup_mutex and memcg_offline_kmem() ends up using
    css_for_each_descendant_pre() without any locking. Fix it by adding rcu
    read locking around it.

    mkdir: cannot create directory `65530': No space left on device
    ===============================
    [ INFO: suspicious RCU usage. ]
    4.6.0-work+ #321 Not tainted
    -------------------------------
    kernel/cgroup.c:4008 cgroup_mutex or RCU read lock required!
    [ 527.243970] other info that might help us debug this:
    [ 527.244715]
    rcu_scheduler_active = 1, debug_locks = 0
    2 locks held by kworker/0:5/1664:
    #0: ("cgroup_destroy"){.+.+..}, at: [<ffffffff81060ab5>] process_one_work+0x165/0x4a0
    #1: ((&css->destroy_work)#3){+.+...}, at: [<ffffffff81060ab5>] process_one_work+0x165/0x4a0
    [ 527.248098] stack backtrace:
    CPU: 0 PID: 1664 Comm: kworker/0:5 Not tainted 4.6.0-work+ #321
    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.9.1-1.fc24 04/01/2014
    Workqueue: cgroup_destroy css_free_work_fn
    Call Trace:
    dump_stack+0x68/0xa1
    lockdep_rcu_suspicious+0xd7/0x110
    css_next_descendant_pre+0x7d/0xb0
    memcg_offline_kmem.part.44+0x4a/0xc0
    mem_cgroup_css_free+0x1ec/0x200
    css_free_work_fn+0x49/0x5e0
    process_one_work+0x1c5/0x4a0
    worker_thread+0x49/0x490
    kthread+0xea/0x100
    ret_from_fork+0x1f/0x40

    Link: http://lkml.kernel.org/r/20160526203018.GG23194@mtj.duckdns.org
    Signed-off-by: Tejun Heo <tj@kernel.org>
    Acked-by: Vladimir Davydov <vdavydov@virtuozzo.com>
    Acked-by: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Michal Hocko <mhocko@kernel.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    mm/memcontrol.c | 3 +++
    1 file changed, 3 insertions(+)

    --- a/mm/memcontrol.c
    +++ b/mm/memcontrol.c
    @@ -2877,6 +2877,7 @@ static void memcg_offline_kmem(struct me
    * ordering is imposed by list_lru_node->lock taken by
    * memcg_drain_all_list_lrus().
    */
    + rcu_read_lock(); /* can be called from css_free w/o cgroup_mutex */
    css_for_each_descendant_pre(css, &memcg->css) {
    child = mem_cgroup_from_css(css);
    BUG_ON(child->kmemcg_id != kmemcg_id);
    @@ -2884,6 +2885,8 @@ static void memcg_offline_kmem(struct me
    if (!memcg->use_hierarchy)
    break;
    }
    + rcu_read_unlock();
    +
    memcg_drain_all_list_lrus(kmemcg_id, parent->kmemcg_id);

    memcg_free_cache_id(kmemcg_id);

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