Re: [BUGFIX] memcg: fix res_counter_read_u64 lock aware (Was Re:[PATCH] oom: handle overflow in mem_cgroup_out_of_memory()

[Andrew Morton]

On Thu, 27 Jan 2011 09:53:42 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:

> res_counter_read_u64 reads u64 value without lock. It's dangerous
> in 32bit environment. This patch adds lock.
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> ---
>  include/linux/res_counter.h |   13 ++++++++++++-
>  kernel/res_counter.c        |    2 +-
>  2 files changed, 13 insertions(+), 2 deletions(-)
> 
> Index: mmotm-0125/include/linux/res_counter.h
> ===================================================================
> --- mmotm-0125.orig/include/linux/res_counter.h
> +++ mmotm-0125/include/linux/res_counter.h
> @@ -68,7 +68,18 @@ struct res_counter {
>   * @pos:     and the offset.
>   */
>  
> -u64 res_counter_read_u64(struct res_counter *counter, int member);
> +u64 res_counter_read_u64_locked(struct res_counter *counter, int member);
> +
> +static inline u64 res_counter_read_u64(struct res_counter *counter, int member)
> +{
> +	unsigned long flags;
> +	u64 ret;
> +
> +	spin_lock_irqsave(&counter->lock, flags);
> +	ret = res_counter_read_u64_locked(counter, member);
> +	spin_unlock_irqrestore(&counter->lock, flags);
> +	return ret;
> +}
>  
>  ssize_t res_counter_read(struct res_counter *counter, int member,
>  		const char __user *buf, size_t nbytes, loff_t *pos,
> Index: mmotm-0125/kernel/res_counter.c
> ===================================================================
> --- mmotm-0125.orig/kernel/res_counter.c
> +++ mmotm-0125/kernel/res_counter.c
> @@ -126,7 +126,7 @@ ssize_t res_counter_read(struct res_coun
>  			pos, buf, s - buf);
>  }
>  
> -u64 res_counter_read_u64(struct res_counter *counter, int member)
> +u64 res_counter_read_u64_locked(struct res_counter *counter, int member)
>  {
>  	return *res_counter_member(counter, member);
>  }

We don't need the lock on 64-bit platforms!

And there's zero benefit to inlining the spin_lock/unlock(), given that
the function will always be making a function call anyway.

See i_size_read() for inspiration.
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