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

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> [2008-12-05 21:24:50]:

> 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. 

A similar race has been reported for cpuset_migrate_mm(), which is
called holding the cgroup_mutex and further calls do_migrate_pages,
which can call reclaim and thus try to acquire cgroup_lock. If we
avoid reclaiming pages with cpuset_migrate_mm(), as the first patch
did, it also solves the reported race.

> 
> Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>

Looks good to me

Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>

-- 
	Balbir

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