Re: [BUGFIX] memcg CPU hotplug lockdep warning fix

[Andrew Morton]

On Fri, 8 Oct 2010 23:19:58 +0530
Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> wrote:

> 
> memcg has lockdep warnings (sleep inside rcu lock)
> 
> From: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>
> 
> Recent move to get_online_cpus() ends up calling get_online_cpus() from
> mem_cgroup_read_stat(). However mem_cgroup_read_stat() is called under rcu
> lock. get_online_cpus() can sleep. The dirty limit patches expose
> this BUG more readily due to their usage of mem_cgroup_page_stat()
> 
> This patch address this issue as identified by lockdep and moves the
> hotplug protection to a higher layer. This might increase the time
> required to hotplug, but not by much.
> 
> Warning messages
> 
> BUG: sleeping function called from invalid context at kernel/cpu.c:62
> in_atomic(): 0, irqs_disabled(): 0, pid: 6325, name: pagetest
> 2 locks held by pagetest/6325:
> #0:  (&mm->mmap_sem){......}, at: [<ffffffff815e9503>]
> do_page_fault+0x27d/0x4a0
> #1:  (rcu_read_lock){......}, at: [<ffffffff811124a1>]
> mem_cgroup_page_stat+0x0/0x23f
> Pid: 6325, comm: pagetest Not tainted 2.6.36-rc5-mm1+ #201
> Call Trace:
> [<ffffffff81041224>] __might_sleep+0x12d/0x131
> [<ffffffff8104f4af>] get_online_cpus+0x1c/0x51
> [<ffffffff8110eedb>] mem_cgroup_read_stat+0x27/0xa3
> [<ffffffff811125d2>] mem_cgroup_page_stat+0x131/0x23f
> [<ffffffff811124a1>] ? mem_cgroup_page_stat+0x0/0x23f
> [<ffffffff810d57c3>] global_dirty_limits+0x42/0xf8
> [<ffffffff810d58b3>] throttle_vm_writeout+0x3a/0xb4
> [<ffffffff810dc2f8>] shrink_zone+0x3e6/0x3f8  
> [<ffffffff81074a35>] ? ktime_get_ts+0xb2/0xbf 
> [<ffffffff810dd1aa>] do_try_to_free_pages+0x106/0x478
> [<ffffffff810dd601>] try_to_free_mem_cgroup_pages+0xe5/0x14c
> [<ffffffff8110f947>] mem_cgroup_hierarchical_reclaim+0x314/0x3a2
> [<ffffffff81111b31>] __mem_cgroup_try_charge+0x29b/0x593
> [<ffffffff8111194a>] ? __mem_cgroup_try_charge+0xb4/0x593
> [<ffffffff81071258>] ? local_clock+0x40/0x59  
> [<ffffffff81009015>] ? sched_clock+0x9/0xd
> [<ffffffff810710d5>] ? sched_clock_local+0x1c/0x82
> [<ffffffff8111398a>] mem_cgroup_charge_common+0x4b/0x76
> [<ffffffff81141469>] ? bio_add_page+0x36/0x38 
> [<ffffffff81113ba9>] mem_cgroup_cache_charge+0x1f4/0x214
> [<ffffffff810cd195>] add_to_page_cache_locked+0x4a/0x148
> ....
> 
> 
> Signed-off-by: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>
> ---
> 
>  mm/memcontrol.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 116fecd..f4c5665 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -578,7 +578,6 @@ static s64 mem_cgroup_read_stat(struct mem_cgroup *mem,
>  	int cpu;
>  	s64 val = 0;
>  
> -	get_online_cpus();
>  	for_each_online_cpu(cpu)
>  		val += per_cpu(mem->stat->count[idx], cpu);
>  #ifdef CONFIG_HOTPLUG_CPU
> @@ -586,7 +585,6 @@ static s64 mem_cgroup_read_stat(struct mem_cgroup *mem,
>  	val += mem->nocpu_base.count[idx];
>  	spin_unlock(&mem->pcp_counter_lock);
>  #endif
> -	put_online_cpus();
>  	return val;
>  }
>  
> @@ -1284,6 +1282,7 @@ s64 mem_cgroup_page_stat(enum mem_cgroup_read_page_stat_item item)
>  	struct mem_cgroup *iter;
>  	s64 value;
>  
> +	get_online_cpus();
>  	rcu_read_lock();
>  	mem = mem_cgroup_from_task(current);
>  	if (mem && !mem_cgroup_is_root(mem)) {
> @@ -1305,6 +1304,7 @@ s64 mem_cgroup_page_stat(enum mem_cgroup_read_page_stat_item item)
>  	} else
>  		value = -EINVAL;
>  	rcu_read_unlock();
> +	put_online_cpus();
>  
>  	return value;
>  }

Confused again.  There's no mem_cgroup_page_stat() in mainline,
linux-next or in any patches in -mm.

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