Re: [PATCH 3/8] CGroup Files: Move the release_agent file to use typed handlers

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 20 Jun 2008 16:44:01 -0700
menage@google.com wrote:

> 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)
> +{
> +	mutex_lock(&cgroup_mutex);
> +	if (cgroup_is_removed(cgrp)) {
> +		mutex_unlock(&cgroup_mutex);
> +		return false;
> +	}
> +	return true;
> +}

I think that if we're going to do this it would be nice to add a
symmetrical cgroup_unlock_live_group()?

Because code like this:

> +	if (!cgroup_lock_live_group(cgrp))
> +		return -ENODEV;
> +	strcpy(cgrp->root->release_agent_path, buffer);
> +	mutex_unlock(&cgroup_mutex);

is a bit WTFish, no?  it forces each caller of cgroup_lock_live_group()
to know about cgroup_lock_live_group() internals.

That would be kind of OKayish if this code was closely localised, but...

> --- 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);
> 

I assume this gets used in another .c file in a  later patch.