Re: [BUGFIX] memcg: avoid deadlock between move charge andtry_charge()

[Daisuke Nishimura]

On Tue, 16 Nov 2010 12:41:17 -0800
Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Tue, 16 Nov 2010 19:17:48 +0900
> Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx> wrote:
> 
> > From: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
> > 
> > __mem_cgroup_try_charge() can be called under down_write(&mmap_sem)(e.g.
> > mlock does it). This means it can cause deadlock if it races with move charge:
> > 
> > Ex.1)
> >                 move charge             |        try charge
> >   --------------------------------------+------------------------------
> >     mem_cgroup_can_attach()             |  down_write(&mmap_sem)
> >       mc.moving_task = current          |    ..
> >       mem_cgroup_precharge_mc()         |  __mem_cgroup_try_charge()
> >         mem_cgroup_count_precharge()    |    prepare_to_wait()
> >           down_read(&mmap_sem)          |    if (mc.moving_task)
> >           -> cannot aquire the lock     |    -> true
> >                                         |      schedule()
> > 
> > Ex.2)
> >                 move charge             |        try charge
> >   --------------------------------------+------------------------------
> >     mem_cgroup_can_attach()             |
> >       mc.moving_task = current          |
> >       mem_cgroup_precharge_mc()         |
> >         mem_cgroup_count_precharge()    |
> >           down_read(&mmap_sem)          |
> >           ..                            |
> >           up_read(&mmap_sem)            |
> >                                         |  down_write(&mmap_sem)
> >     mem_cgroup_move_task()              |    ..
> >       mem_cgroup_move_charge()          |  __mem_cgroup_try_charge()
> >         down_read(&mmap_sem)            |    prepare_to_wait()
> >         -> cannot aquire the lock       |    if (mc.moving_task)
> >                                         |    -> true
> >                                         |      schedule()
> > 
> > To avoid this deadlock, we do all the move charge works (both can_attach() and
> > attach()) under one mmap_sem section.
> > And after this patch, we set/clear mc.moving_task outside mc.lock, because we
> > use the lock only to check mc.from/to.
> > 
> > Signed-off-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
> 
> I put this in the send-to-Linus-in-about-a-week queue.
> 
> > Cc: <stable@xxxxxxxxxx>
> 
> The patch doesn't apply well to 2.6.36 so if we do want it backported
> then please prepare a tested backport for the -stable guys?
> 
O.K.
I'll test a backported patch for 2.6.36.y and send it after this is merged into mainline.

Thanks,
Daisuke Nishimura.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/