Re: [RFC][PATCH -mmotm 3/4] memcg: avoid dead lock caused by racebetween oom and cpuset_attach

[KAMEZAWA Hiroyuki]

Daisuke Nishimura said:
> 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).
>
good catch.

> 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().
agree to this.

>   - define new mutex (memcg_tasklist) and serialize mem_cgroup_move_task()
>     (->attach handler of memory cgroup) and mem_cgroup_out_of_memory.
>
Hmm...seems temporal fix (and adding new global lock...)
But ok, we need fix. revist this later.

Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxx,com>


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