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Date: Wed, 29 Feb 2012 20:00:46 +0100
From: Johannes Weiner <hannes@cmpxchg.org>
To: Hugh Dickins <hughd@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Konstantin Khlebnikov <khlebnikov@openvz.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 3.3] memcg: fix deadlock by inverting lrucare nesting
Message-ID: <20120229190046.GC1673@cmpxchg.org>
References: <alpine.LSU.2.00.1202282121160.4875@eggly.anvils>
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On Tue, Feb 28, 2012 at 09:25:02PM -0800, Hugh Dickins 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.
> 
> Signed-off-by: Hugh Dickins <hughd@google.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>
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