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Subject[PATCH -mmotm 3/4] memcg: avoid dead lock caused by race between oom and cpuset_attach
mpol_rebind_mm(), which can be called from cpuset_attach(), does down_write(mm->mmap_sem).
This means down_write(mm->mmap_sem) can be called under cgroup_mutex.

OTOH, page fault path does down_read(mm->mmap_sem) and calls mem_cgroup_try_charge_xxx(),
which may eventually calls mem_cgroup_out_of_memory(). And mem_cgroup_out_of_memory()
calls cgroup_lock().
This means cgroup_lock() can be called under down_read(mm->mmap_sem).

If those two paths race, dead lock can happen.

This patch avoid this dead lock by:
- remove cgroup_lock() from mem_cgroup_out_of_memory().
- define new mutex (memcg_tasklist) and serialize mem_cgroup_move_task()
(->attach handler of memory cgroup) and mem_cgroup_out_of_memory.

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu,com>
Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>
---
mm/memcontrol.c | 5 +++++
mm/oom_kill.c | 2 --
2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 9877b03..fec4fc3 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -51,6 +51,7 @@ static int really_do_swap_account __initdata = 1; /* for remember boot option*/
#define do_swap_account (0)
#endif

+static DEFINE_MUTEX(memcg_tasklist); /* can be hold under cgroup_mutex */

/*
* Statistics for memory cgroup.
@@ -797,7 +798,9 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,

if (!nr_retries--) {
if (oom) {
+ mutex_lock(&memcg_tasklist);
mem_cgroup_out_of_memory(mem_over_limit, gfp_mask);
+ mutex_unlock(&memcg_tasklist);
mem_over_limit->last_oom_jiffies = jiffies;
}
goto nomem;
@@ -2173,10 +2176,12 @@ static void mem_cgroup_move_task(struct cgroup_subsys *ss,
struct cgroup *old_cont,
struct task_struct *p)
{
+ mutex_lock(&memcg_tasklist);
/*
* FIXME: It's better to move charges of this process from old
* memcg to new memcg. But it's just on TODO-List now.
*/
+ mutex_unlock(&memcg_tasklist);
}

struct cgroup_subsys mem_cgroup_subsys = {
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index fd150e3..40ba050 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -429,7 +429,6 @@ void mem_cgroup_out_of_memory(struct mem_cgroup *mem, gfp_t gfp_mask)
unsigned long points = 0;
struct task_struct *p;

- cgroup_lock();
read_lock(&tasklist_lock);
retry:
p = select_bad_process(&points, mem);
@@ -444,7 +443,6 @@ retry:
goto retry;
out:
read_unlock(&tasklist_lock);
- cgroup_unlock();
}
#endif


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