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Message-ID: <20080624155634.GB4993@us.ibm.com>
Date: Tue, 24 Jun 2008 10:56:34 -0500
From: "Serge E. Hallyn" <serue@...ibm.com>
To: menage@...gle.com
Cc: pj@....com, xemul@...nvz.org, balbir@...ibm.com, serue@...ibm.com,
akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
containers@...ts.linux-foundation.org
Subject: Re: [PATCH 3/8] CGroup Files: Move the release_agent file to use
typed handlers
Quoting menage@...gle.com (menage@...gle.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@...gle.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@...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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