Re: [PATCH 1/5] cgroup: Fix an RCU warning in cgroup_path()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Apr 22, 2010 at 05:29:24PM +0800, Li Zefan wrote:
> with CONFIG_PROVE_RCU=y, a warning can be triggered:
> 
>   # mount -t cgroup -o debug xxx /mnt
>   # cat /proc/$$/cgroup
> 
> ...
> kernel/cgroup.c:1649 invoked rcu_dereference_check() without protection!
> ...
> 
> This is a false-positive, because cgroup_path() can be called
> with either rcu_read_lock() held or cgroup_mutex held.

Queued for 2.6.34, thank you Li!

							Thanx, Paul

> Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
> ---
>  kernel/cgroup.c |   12 +++++++++---
>  1 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index e2769e1..4ca928d 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -1646,7 +1646,9 @@ static inline struct cftype *__d_cft(struct dentry *dentry)
>  int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)
>  {
>  	char *start;
> -	struct dentry *dentry = rcu_dereference(cgrp->dentry);
> +	struct dentry *dentry = rcu_dereference_check(cgrp->dentry,
> +						      rcu_read_lock_held() ||
> +						      cgroup_lock_is_held());
> 
>  	if (!dentry || cgrp == dummytop) {
>  		/*
> @@ -1662,13 +1664,17 @@ int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)
>  	*--start = '\0';
>  	for (;;) {
>  		int len = dentry->d_name.len;
> +
>  		if ((start -= len) < buf)
>  			return -ENAMETOOLONG;
> -		memcpy(start, cgrp->dentry->d_name.name, len);
> +		memcpy(start, dentry->d_name.name, len);
>  		cgrp = cgrp->parent;
>  		if (!cgrp)
>  			break;
> -		dentry = rcu_dereference(cgrp->dentry);
> +
> +		dentry = rcu_dereference_check(cgrp->dentry,
> +					       rcu_read_lock_held() ||
> +					       cgroup_lock_is_held());
>  		if (!cgrp->parent)
>  			continue;
>  		if (--start < buf)
> -- 
> 1.6.3
>