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SubjectRe: [PATCH 3.3] memcg: fix deadlock by inverting lrucare nesting
On Tue, 28 Feb 2012 21:25:02 -0800 (PST)
Hugh Dickins <hughd@google.com> wrote:

> We have forgotten the rules of lock nesting: the irq-safe ones must be
> taken inside the non-irq-safe ones, otherwise we are open to deadlock:
>
> CPU0 CPU1
> ---- ----
> lock(&(&pc->lock)->rlock);
> local_irq_disable();
> lock(&(&zone->lru_lock)->rlock);
> lock(&(&pc->lock)->rlock);
> <Interrupt>
> lock(&(&zone->lru_lock)->rlock);
>
> To check a different locking issue, I happened to add a spin_lock to
> memcg's bit_spin_lock in lock_page_cgroup(), and lockdep very quickly
> complained about __mem_cgroup_commit_charge_lrucare() (on CPU1 above).
>
> So delete __mem_cgroup_commit_charge_lrucare(), passing a bool lrucare
> to __mem_cgroup_commit_charge() instead, taking zone->lru_lock under
> lock_page_cgroup() in the lrucare case.
>
> The original was using spin_lock_irqsave, but we'd be in more trouble
> if it were ever called at interrupt time: unconditional _irq is enough.
> And ClearPageLRU before del from lru, SetPageLRU before add to lru: no
> strong reason, but that is the ordering used consistently elsewhere.

This patch makes rather a mess of "memcg: remove PCG_CACHE page_cgroup
flag".

--- mm/memcontrol.c~memcg-remove-pcg_cache-page_cgroup-flag
+++ mm/memcontrol.c
@@ -2410,6 +2414,8 @@
struct page_cgroup *pc,
enum charge_type ctype)
{
+ bool anon;
+
lock_page_cgroup(pc);
if (unlikely(PageCgroupUsed(pc))) {
unlock_page_cgroup(pc);
@@ -2429,21 +2435,14 @@
* See mem_cgroup_add_lru_list(), etc.
*/
smp_wmb();
- switch (ctype) {
- case MEM_CGROUP_CHARGE_TYPE_CACHE:
- case MEM_CGROUP_CHARGE_TYPE_SHMEM:
- SetPageCgroupCache(pc);
- SetPageCgroupUsed(pc);
- break;
- case MEM_CGROUP_CHARGE_TYPE_MAPPED:
- ClearPageCgroupCache(pc);
- SetPageCgroupUsed(pc);
- break;
- default:
- break;
- }

- mem_cgroup_charge_statistics(memcg, PageCgroupCache(pc), nr_pages);
+ SetPageCgroupUsed(pc);
+ if (ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED)
+ anon = true;
+ else
+ anon = false;
+
+ mem_cgroup_charge_statistics(memcg, anon, nr_pages);
unlock_page_cgroup(pc);
WARN_ON_ONCE(PageLRU(page));
/*
I did it this way:

static void __mem_cgroup_commit_charge(struct mem_cgroup *memcg,
struct page *page,
unsigned int nr_pages,
struct page_cgroup *pc,
enum charge_type ctype,
bool lrucare)
{
struct zone *uninitialized_var(zone);
bool was_on_lru = false;
bool anon;
lock_page_cgroup(pc);
if (unlikely(PageCgroupUsed(pc))) {
unlock_page_cgroup(pc);
__mem_cgroup_cancel_charge(memcg, nr_pages);
return;
}
/*
* we don't need page_cgroup_lock about tail pages, becase they are not
* accessed by any other context at this point.
*/
/*
* In some cases, SwapCache and FUSE(splice_buf->radixtree), the page
* may already be on some other mem_cgroup's LRU. Take care of it.
*/
if (lrucare) {
zone = page_zone(page);
spin_lock_irq(&zone->lru_lock);
if (PageLRU(page)) {
ClearPageLRU(page);
del_page_from_lru_list(zone, page, page_lru(page));
was_on_lru = true;
}
}
pc->mem_cgroup = memcg;
/*
* We access a page_cgroup asynchronously without lock_page_cgroup().
* Especially when a page_cgroup is taken from a page, pc->mem_cgroup
* is accessed after testing USED bit. To make pc->mem_cgroup visible
* before USED bit, we need memory barrier here.
* See mem_cgroup_add_lru_list(), etc.
*/
smp_wmb();
SetPageCgroupUsed(pc);
if (lrucare) {
if (was_on_lru) {
VM_BUG_ON(PageLRU(page));
SetPageLRU(page);
add_page_to_lru_list(zone, page, page_lru(page));
}
spin_unlock_irq(&zone->lru_lock);
}
if (ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED)
anon = true;
else
anon = false;
mem_cgroup_charge_statistics(memcg, anon, nr_pages);
unlock_page_cgroup(pc);

/*
* "charge_statistics" updated event counter. Then, check it.
* Insert ancestor (and ancestor's ancestors), to softlimit RB-tree.
* if they exceeds softlimit.
*/
memcg_check_events(memcg, page);
}


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