On Wed 24-09-14 22:40:55, Johannes Weiner wrote: > Argh, buggy css_put() against the root. Hand grenades, everywhere. > Update: > > --- > From 9b0b4d72d71cd8acd7aaa58d2006c751decc8739 Mon Sep 17 00:00:00 2001 > From: Johannes Weiner <hannes@xxxxxxxxxxx> > Date: Wed, 24 Sep 2014 22:00:20 -0400 > Subject: [patch] mm: memcontrol: do not iterate uninitialized memcgs > > The cgroup iterators yield css objects that have not yet gone through > css_online(), but they are not complete memcgs at this point and so > the memcg iterators should not return them. d8ad30559715 ("mm/memcg: > iteration skip memcgs not yet fully initialized") set out to implement > exactly this, but it uses CSS_ONLINE, a cgroup-internal flag that does > not meet the ordering requirements for memcg, and so we still may see > partially initialized memcgs from the iterators. I do not see how would this happen. CSS_ONLINE is set after css_online callback returns and mem_cgroup_css_online ends the core initialization with mutex_unlock which should provide sufficient memory ordering requirements (kmem is not covered but activate_kmem_mutex kmem.tcp by proto_list_mutex). So the worst thing that might happen is that we miss an already initialized memcg but that shouldn't matter because such a memcg doesn't contain any tasks nor memory. memcg_has_children doesn't rely on our iterators so important parts will not miss anything. So I do not see any bug right now. The flag abuse is another story and I do agree we should use proper memcg specific synchronization here as explained by Tejun in other email. > The cgroup core can not reasonably provide a clear answer on whether > the object around the css has been fully initialized, as that depends > on controller-specific locking and lifetime rules. Thus, introduce a > memcg-specific flag that is set after the memcg has been initialized > in css_online(), and read before mem_cgroup_iter() callers access the > memcg members. > > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> With updated changelog Acked-by: Michal Hocko <mhocko@xxxxxxx> > Cc: <stable@xxxxxxxxxxxxxxx> [3.12+] This is not necessary IMO > --- > mm/memcontrol.c | 36 +++++++++++++++++++++++++++++++----- > 1 file changed, 31 insertions(+), 5 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 306b6470784c..bafdac0f724e 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -292,6 +292,9 @@ struct mem_cgroup { > /* vmpressure notifications */ > struct vmpressure vmpressure; > > + /* css_online() has been completed */ > + bool initialized; > + > /* > * the counter to account for mem+swap usage. > */ > @@ -1090,10 +1093,23 @@ skip_node: > * skipping css reference should be safe. > */ > if (next_css) { > - if ((next_css == &root->css) || > - ((next_css->flags & CSS_ONLINE) && > - css_tryget_online(next_css))) > - return mem_cgroup_from_css(next_css); > + struct mem_cgroup *memcg = mem_cgroup_from_css(next_css); > + > + if (next_css == &root->css) > + return memcg; > + > + if (css_tryget_online(next_css)) { > + if (memcg->initialized) { > + /* > + * Make sure the caller's accesses to > + * the memcg members are issued after > + * we see this flag set. > + */ > + smp_rmb(); > + return memcg; > + } > + css_put(next_css); > + } > > prev_css = next_css; > goto skip_node; > @@ -5413,6 +5429,7 @@ mem_cgroup_css_online(struct cgroup_subsys_state *css) > { > struct mem_cgroup *memcg = mem_cgroup_from_css(css); > struct mem_cgroup *parent = mem_cgroup_from_css(css->parent); > + int ret; > > if (css->id > MEM_CGROUP_ID_MAX) > return -ENOSPC; > @@ -5449,7 +5466,16 @@ mem_cgroup_css_online(struct cgroup_subsys_state *css) > } > mutex_unlock(&memcg_create_mutex); > > - return memcg_init_kmem(memcg, &memory_cgrp_subsys); > + ret = memcg_init_kmem(memcg, &memory_cgrp_subsys); > + if (ret) > + return ret; > + > + /* Make sure the initialization is visible before the flag */ > + smp_wmb(); > + > + memcg->initialized = true; > + > + return 0; > } > > /* > -- > 2.1.0 > -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>