Re: [PATCH RFC] memcg: fix drain_all_stock crash

[KAMEZAWA Hiroyuki]

On Tue, 9 Aug 2011 11:31:50 +0200
Michal Hocko <mhocko@xxxxxxx> wrote:

> What do you think about the half backed patch bellow? I didn't manage to
> test it yet but I guess it should help. I hate asymmetry of drain_lock
> locking (it is acquired somewhere else than it is released which is
> not). I will think about a nicer way how to do it.
> Maybe I should also split the rcu part in a separate patch.
> 
> What do you think?


I'd like to revert 8521fc50 first and consider total design change
rather than ad-hoc fix.

Personally, I don't like to have spin-lock in per-cpu area.


Thanks,
-Kame

> ---
> From 26c2cdc55aa14ec4a54e9c8e2c8b9072c7cb8e28 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@xxxxxxx>
> Date: Tue, 9 Aug 2011 10:53:28 +0200
> Subject: [PATCH] memcg: fix drain_all_stock crash
> 
> 8521fc50 (memcg: get rid of percpu_charge_mutex lock) introduced a crash
> in sync mode when we are about to check whether we have to wait for the
> work because we are calling mem_cgroup_same_or_subtree without checking
> FLUSHING_CACHED_CHARGE before so we can dereference already cleaned
> cache (the simplest case would be when we drain the local cache).
> 
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
> IP: [<ffffffff81083b70>] css_is_ancestor+0x20/0x70
> PGD 4ae7a067 PUD 4adc4067 PMD 0
> Oops: 0000 [#1] PREEMPT SMP
> CPU 0
> Pid: 19677, comm: rmdir Tainted: G        W   3.0.0-mm1-00188-gf38d32b #35 ECS MCP61M-M3/MCP61M-M3
> RIP: 0010:[<ffffffff81083b70>]  [<ffffffff81083b70>] css_is_ancestor+0x20/0x70
> RSP: 0018:ffff880077b09c88  EFLAGS: 00010202
> RAX: ffff8800781bb310 RBX: 0000000000000000 RCX: 000000000000003e
> RDX: 0000000000000000 RSI: ffff8800779f7c00 RDI: 0000000000000000
> RBP: ffff880077b09c98 R08: ffffffff818a4e88 R09: 0000000000000000
> R10: 0000000000000000 R11: dead000000100100 R12: ffff8800779f7c00
> R13: ffff8800779f7c00 R14: 0000000000000000 R15: ffff88007bc0eb80
> FS:  00007f5d689ec720(0000) GS:ffff88007bc00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> CR2: 0000000000000018 CR3: 000000004ad57000 CR4: 00000000000006f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> Process rmdir (pid: 19677, threadinfo ffff880077b08000, task ffff8800781bb310)
> Stack:
>  ffffffff818a4e88 000000000000eb80 ffff880077b09ca8 ffffffff810feba3
>  ffff880077b09d08 ffffffff810feccf ffff880077b09cf8 0000000000000001
>  ffff88007bd0eb80 0000000000000001 ffff880077af2000 0000000000000000
> Call Trace:
>  [<ffffffff810feba3>] mem_cgroup_same_or_subtree+0x33/0x40
>  [<ffffffff810feccf>] drain_all_stock+0x11f/0x170
>  [<ffffffff81103211>] mem_cgroup_force_empty+0x231/0x6d0
>  [<ffffffff81111872>] ? path_put+0x22/0x30
>  [<ffffffff8111c925>] ? __d_lookup+0xb5/0x170
>  [<ffffffff811036c4>] mem_cgroup_pre_destroy+0x14/0x20
>  [<ffffffff81080559>] cgroup_rmdir+0xb9/0x500
>  [<ffffffff81063990>] ? abort_exclusive_wait+0xb0/0xb0
>  [<ffffffff81114d26>] vfs_rmdir+0x86/0xe0
>  [<ffffffff811233d3>] ? mnt_want_write+0x43/0x80
>  [<ffffffff81114e7b>] do_rmdir+0xfb/0x110
>  [<ffffffff81114ea6>] sys_rmdir+0x16/0x20
>  [<ffffffff8154d76b>] system_call_fastpath+0x16/0x1b
> 
> Testing FLUSHING_CACHED_CHARGE before dereferencing is still not enough
> because then we still might see mem == NULL so we have to check it
> before dereferencing.
> We have to do all stock checking under FLUSHING_CACHED_CHARGE bit lock
> so it is much easier to use a spin_lock instead. Let's also add a flag
> (under_drain) that draining in progress so that concurrent callers do
> not have to wait on the lock pointlessly.
> 
> Finally we do not make sure that the mem still exists. It could have
> been removed in the meantime:
> 	CPU0			CPU1			     CPU2
> mem=stock->cached
> stock->cached=NULL
> 			      clear_bit
> 							test_and_set_bit
> test_bit()		      ...
> <preempted>		mem_cgroup_destroy
> use after free
> 
> `...' is actually quite a bunch of work to do so the race is not very
> probable. The important thing, though, is that cgroup_subsys->destroy
> (mem_cgroup_destroy) is called after synchronize_rcu so we can protect
> by calling rcu_read_lock when dereferencing cached mem.
> 
> TODO:
> - check if under_drain needs some memory barriers
> - check the hotplug path (can we wait on spinlock?)
> - better changelog
> - do some testing
> 
> Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
> Reported-by: Johannes Weiner <jweiner@xxxxxxxxxx>
> ---
>  mm/memcontrol.c |   67 +++++++++++++++++++++++++++++++++++++++++-------------
>  1 files changed, 51 insertions(+), 16 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index f4ec4e7..e34f9fd 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2087,8 +2087,8 @@ struct memcg_stock_pcp {
>  	struct mem_cgroup *cached; /* this never be root cgroup */
>  	unsigned int nr_pages;
>  	struct work_struct work;
> -	unsigned long flags;
> -#define FLUSHING_CACHED_CHARGE	(0)
> +	spinlock_t drain_lock; /* protects from parallel draining */
> +	bool under_drain;
>  };
>  static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock);
>  
> @@ -2114,6 +2114,7 @@ static bool consume_stock(struct mem_cgroup *mem)
>  
>  /*
>   * Returns stocks cached in percpu to res_counter and reset cached information.
> + * Do not call this directly - use drain_local_stock instead.
>   */
>  static void drain_stock(struct memcg_stock_pcp *stock)
>  {
> @@ -2133,12 +2134,16 @@ static void drain_stock(struct memcg_stock_pcp *stock)
>  /*
>   * This must be called under preempt disabled or must be called by
>   * a thread which is pinned to local cpu.
> + * Parameter is not used.
> + * Assumes stock->drain_lock held.
>   */
>  static void drain_local_stock(struct work_struct *dummy)
>  {
>  	struct memcg_stock_pcp *stock = &__get_cpu_var(memcg_stock);
>  	drain_stock(stock);
> -	clear_bit(FLUSHING_CACHED_CHARGE, &stock->flags);
> +
> +	stock->under_drain = false;
> +	spin_unlock(&stock->drain_lock);
>  }
>  
>  /*
> @@ -2150,7 +2155,9 @@ static void refill_stock(struct mem_cgroup *mem, unsigned int nr_pages)
>  	struct memcg_stock_pcp *stock = &get_cpu_var(memcg_stock);
>  
>  	if (stock->cached != mem) { /* reset if necessary */
> -		drain_stock(stock);
> +		spin_lock(&stock->drain_lock);
> +		stock->under_drain = true;
> +		drain_local_stock(NULL);
>  		stock->cached = mem;
>  	}
>  	stock->nr_pages += nr_pages;
> @@ -2179,17 +2186,27 @@ static void drain_all_stock(struct mem_cgroup *root_mem, bool sync)
>  		struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
>  		struct mem_cgroup *mem;
>  
> +		/*
> +		 * make sure we are not waiting when somebody already drains
> +		 * the cache.
> +		 */
> +		if (!spin_trylock(&stock->drain_lock)) {
> +			if (stock->under_drain)
> +				continue;
> +			spin_lock(&stock->drain_lock);
> +		}
>  		mem = stock->cached;
> -		if (!mem || !stock->nr_pages)
> +		if (!mem || !stock->nr_pages ||
> +				!mem_cgroup_same_or_subtree(root_mem, mem)) {
> +			spin_unlock(&stock->drain_lock);
>  			continue;
> -		if (!mem_cgroup_same_or_subtree(root_mem, mem))
> -			continue;
> -		if (!test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags)) {
> -			if (cpu == curcpu)
> -				drain_local_stock(&stock->work);
> -			else
> -				schedule_work_on(cpu, &stock->work);
>  		}
> +
> +		stock->under_drain = true;
> +		if (cpu == curcpu)
> +			drain_local_stock(&stock->work);
> +		else
> +			schedule_work_on(cpu, &stock->work);
>  	}
>  
>  	if (!sync)
> @@ -2197,8 +2214,20 @@ static void drain_all_stock(struct mem_cgroup *root_mem, bool sync)
>  
>  	for_each_online_cpu(cpu) {
>  		struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
> -		if (mem_cgroup_same_or_subtree(root_mem, stock->cached) &&
> -				test_bit(FLUSHING_CACHED_CHARGE, &stock->flags))
> +		struct mem_cgroup *mem;
> +		bool wait_for_drain = false;
> +
> +		/*
> +		 * we have to be careful about parallel group destroying
> +		 * (mem_cgroup_destroy) which is derefered after sychronize_rcu
> +		 */
> +		rcu_read_lock();
> +		mem = stock->cached;
> +		wait_for_drain = stock->under_drain &&
> +			mem && mem_cgroup_same_or_subtree(root_mem, mem);
> +		rcu_read_unlock();
> +
> +		if (wait_for_drain)
>  			flush_work(&stock->work);
>  	}
>  out:
> @@ -2278,8 +2307,12 @@ static int __cpuinit memcg_cpu_hotplug_callback(struct notifier_block *nb,
>  	for_each_mem_cgroup_all(iter)
>  		mem_cgroup_drain_pcp_counter(iter, cpu);
>  
> -	stock = &per_cpu(memcg_stock, cpu);
> -	drain_stock(stock);
> +	if (!spin_trylock(&stock->drain_lock)) {
> +		if (stock->under_drain)
> +			return NOTIFY_OK;
> +		spin_lock(&stock->drain_lock);
> +	}
> +	drain_local_stock(NULL);
>  	return NOTIFY_OK;
>  }
>  
> @@ -5068,6 +5101,8 @@ mem_cgroup_create(struct cgroup_subsys *ss, struct cgroup *cont)
>  			struct memcg_stock_pcp *stock =
>  						&per_cpu(memcg_stock, cpu);
>  			INIT_WORK(&stock->work, drain_local_stock);
> +			stock->under_drain = false;
> +			spin_lock_init(&stock->drain_lock);
>  		}
>  		hotcpu_notifier(memcg_cpu_hotplug_callback, 0);
>  	} else {
> -- 
> 1.7.5.4
> 
> 
> -- 
> Michal Hocko
> SUSE Labs
> SUSE LINUX s.r.o.
> Lihovarska 1060/12
> 190 00 Praha 9    
> Czech Republic
> 

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