Quoting menage@google.com (menage@google.com):
> Adds cgroup_release_agent_write() and cgroup_release_agent_show()
> methods to handle writing/reading the path to a cgroup hierarchy's
> release agent. As a result, cgroup_common_file_read() is now unnecessary.
> 
> As part of the change, a previously-tolerated race in
> cgroup_release_agent() is avoided by copying the current
> release_agent_path prior to calling call_usermode_helper().
> 
> Signed-off-by: Paul Menage <menage@google.com>
> 
> ---
>  include/linux/cgroup.h |    2 
>  kernel/cgroup.c        |  125 ++++++++++++++++++++++---------------------------
>  2 files changed, 59 insertions(+), 68 deletions(-)
> 
> Index: cws-2.6.26-rc5-mm3/kernel/cgroup.c
> ===================================================================
> --- cws-2.6.26-rc5-mm3.orig/kernel/cgroup.c
> +++ cws-2.6.26-rc5-mm3/kernel/cgroup.c
> @@ -89,11 +89,7 @@ struct cgroupfs_root {
>  	/* Hierarchy-specific flags */
>  	unsigned long flags;
> 
> -	/* The path to use for release notifications. No locking
> -	 * between setting and use - so if userspace updates this
> -	 * while child cgroups exist, you could miss a
> -	 * notification. We ensure that it's always a valid
> -	 * NUL-terminated string */
> +	/* The path to use for release notifications. */
>  	char release_agent_path[PATH_MAX];
>  };
> 
> @@ -1329,6 +1325,45 @@ enum cgroup_filetype {
>  	FILE_RELEASE_AGENT,
>  };
> 
> +/**
> + * cgroup_lock_live_group - take cgroup_mutex and check that cgrp is alive.
> + * @cgrp: the cgroup to be checked for liveness
> + *
> + * Returns true (with lock held) on success, or false (with no lock
> + * held) on failure.
> + */
> +int cgroup_lock_live_group(struct cgroup *cgrp)

Would seem more consistent to call the return type bool, but otherwise
this patch looks good.

Acked-by: Serge Hallyn <serue@us.ibm.com>

thanks,
-serge

> +{
> +	mutex_lock(&cgroup_mutex);
> +	if (cgroup_is_removed(cgrp)) {
> +		mutex_unlock(&cgroup_mutex);
> +		return false;
> +	}
> +	return true;
> +}
> +
> +static int cgroup_release_agent_write(struct cgroup *cgrp, struct cftype *cft,
> +				      const char *buffer)
> +{
> +	BUILD_BUG_ON(sizeof(cgrp->root->release_agent_path) < PATH_MAX);
> +	if (!cgroup_lock_live_group(cgrp))
> +		return -ENODEV;
> +	strcpy(cgrp->root->release_agent_path, buffer);
> +	mutex_unlock(&cgroup_mutex);
> +	return 0;
> +}
> +
> +static int cgroup_release_agent_show(struct cgroup *cgrp, struct cftype *cft,
> +				     struct seq_file *seq)
> +{
> +	if (!cgroup_lock_live_group(cgrp))
> +		return -ENODEV;
> +	seq_puts(seq, cgrp->root->release_agent_path);
> +	seq_putc(seq, '\n');
> +	mutex_unlock(&cgroup_mutex);
> +	return 0;
> +}
> +
>  static ssize_t cgroup_write_X64(struct cgroup *cgrp, struct cftype *cft,
>  				struct file *file,
>  				const char __user *userbuf,
> @@ -1443,10 +1478,6 @@ static ssize_t cgroup_common_file_write(
>  		else
>  			clear_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);
>  		break;
> -	case FILE_RELEASE_AGENT:
> -		BUILD_BUG_ON(sizeof(cgrp->root->release_agent_path) < PATH_MAX);
> -		strcpy(cgrp->root->release_agent_path, buffer);
> -		break;
>  	default:
>  		retval = -EINVAL;
>  		goto out2;
> @@ -1506,49 +1537,6 @@ static ssize_t cgroup_read_s64(struct cg
>  	return simple_read_from_buffer(buf, nbytes, ppos, tmp, len);
>  }
> 
> -static ssize_t cgroup_common_file_read(struct cgroup *cgrp,
> -					  struct cftype *cft,
> -					  struct file *file,
> -					  char __user *buf,
> -					  size_t nbytes, loff_t *ppos)
> -{
> -	enum cgroup_filetype type = cft->private;
> -	char *page;
> -	ssize_t retval = 0;
> -	char *s;
> -
> -	if (!(page = (char *)__get_free_page(GFP_KERNEL)))
> -		return -ENOMEM;
> -
> -	s = page;
> -
> -	switch (type) {
> -	case FILE_RELEASE_AGENT:
> -	{
> -		struct cgroupfs_root *root;
> -		size_t n;
> -		mutex_lock(&cgroup_mutex);
> -		root = cgrp->root;
> -		n = strnlen(root->release_agent_path,
> -			    sizeof(root->release_agent_path));
> -		n = min(n, (size_t) PAGE_SIZE);
> -		strncpy(s, root->release_agent_path, n);
> -		mutex_unlock(&cgroup_mutex);
> -		s += n;
> -		break;
> -	}
> -	default:
> -		retval = -EINVAL;
> -		goto out;
> -	}
> -	*s++ = '\n';
> -
> -	retval = simple_read_from_buffer(buf, nbytes, ppos, page, s - page);
> -out:
> -	free_page((unsigned long)page);
> -	return retval;
> -}
> -
>  static ssize_t cgroup_file_read(struct file *file, char __user *buf,
>  				   size_t nbytes, loff_t *ppos)
>  {
> @@ -1606,6 +1594,7 @@ static int cgroup_seqfile_release(struct
> 
>  static struct file_operations cgroup_seqfile_operations = {
>  	.read = seq_read,
> +	.write = cgroup_file_write,
>  	.llseek = seq_lseek,
>  	.release = cgroup_seqfile_release,
>  };
> @@ -2283,8 +2272,9 @@ static struct cftype files[] = {
> 
>  static struct cftype cft_release_agent = {
>  	.name = "release_agent",
> -	.read = cgroup_common_file_read,
> -	.write = cgroup_common_file_write,
> +	.read_seq_string = cgroup_release_agent_show,
> +	.write_string = cgroup_release_agent_write,
> +	.max_write_len = PATH_MAX,
>  	.private = FILE_RELEASE_AGENT,
>  };
> 
> @@ -3113,27 +3103,24 @@ static void cgroup_release_agent(struct 
>  	while (!list_empty(&release_list)) {
>  		char *argv[3], *envp[3];
>  		int i;
> -		char *pathbuf;
> +		char *pathbuf = NULL, *agentbuf = NULL;
>  		struct cgroup *cgrp = list_entry(release_list.next,
>  						    struct cgroup,
>  						    release_list);
>  		list_del_init(&cgrp->release_list);
>  		spin_unlock(&release_list_lock);
>  		pathbuf = kmalloc(PAGE_SIZE, GFP_KERNEL);
> -		if (!pathbuf) {
> -			spin_lock(&release_list_lock);
> -			continue;
> -		}
> -
> -		if (cgroup_path(cgrp, pathbuf, PAGE_SIZE) < 0) {
> -			kfree(pathbuf);
> -			spin_lock(&release_list_lock);
> -			continue;
> -		}
> +		if (!pathbuf)
> +			goto continue_free;
> +		if (cgroup_path(cgrp, pathbuf, PAGE_SIZE) < 0)
> +			goto continue_free;
> +		agentbuf = kstrdup(cgrp->root->release_agent_path, GFP_KERNEL);
> +		if (!agentbuf)
> +			goto continue_free;
> 
>  		i = 0;
> -		argv[i++] = cgrp->root->release_agent_path;
> -		argv[i++] = (char *)pathbuf;
> +		argv[i++] = agentbuf;
> +		argv[i++] = pathbuf;
>  		argv[i] = NULL;
> 
>  		i = 0;
> @@ -3147,8 +3134,10 @@ static void cgroup_release_agent(struct 
>  		 * be a slow process */
>  		mutex_unlock(&cgroup_mutex);
>  		call_usermodehelper(argv[0], argv, envp, UMH_WAIT_EXEC);
> -		kfree(pathbuf);
>  		mutex_lock(&cgroup_mutex);
> + continue_free:
> +		kfree(pathbuf);
> +		kfree(agentbuf);
>  		spin_lock(&release_list_lock);
>  	}
>  	spin_unlock(&release_list_lock);
> Index: cws-2.6.26-rc5-mm3/include/linux/cgroup.h
> ===================================================================
> --- cws-2.6.26-rc5-mm3.orig/include/linux/cgroup.h
> +++ cws-2.6.26-rc5-mm3/include/linux/cgroup.h
> @@ -295,6 +295,8 @@ int cgroup_add_files(struct cgroup *cgrp
> 
>  int cgroup_is_removed(const struct cgroup *cgrp);
> 
> +int cgroup_lock_live_group(struct cgroup *cgrp);
> +
>  int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen);
> 
>  int cgroup_task_count(const struct cgroup *cgrp);
> 
> --
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