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cgroup: combine cgroup_mutex locking and offline css draining

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cgroup_drain_offline() is used to wait for csses being offlined to
uninstall itself from cgroup->subsys[] array so that new csses can be
installed.  The function's only user, cgroup_subtree_control_write(),
calls it after performing some checks and restarts the whole process
via restart_syscall() if draining has to release cgroup_mutex to wait.

This can be simplified by draining before other synchronized
operations so that there's nothing to restart.  This patch converts
cgroup_drain_offline() to cgroup_lock_and_drain_offline() which
performs both locking and draining and updates cgroup_kn_lock_live()
use it instead of cgroup_mutex() if requested.  This combined locking
and draining operations are easier to use and less error-prone.

While at it, add WARNs in control_apply functions which triggers if
the subtree isn't properly drained.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>
This commit is contained in:
Tejun Heo 2016-03-03 09:58:00 -05:00
parent f7b2814bb9
commit 945ba19968
1 changed files with 27 additions and 28 deletions
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55
kernel/cgroup.c
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@ -220,6 +220,7 @@ static struct cftype cgroup_dfl_base_files[];
static struct cftype cgroup_legacy_base_files[];
static int rebind_subsystems(struct cgroup_root *dst_root, u16 ss_mask);
static void cgroup_lock_and_drain_offline(struct cgroup *cgrp);
static void css_task_iter_advance(struct css_task_iter *it);
static int cgroup_destroy_locked(struct cgroup *cgrp);
static struct cgroup_subsys_state *css_create(struct cgroup *cgrp,
@ -1391,19 +1392,22 @@ static void cgroup_kn_unlock(struct kernfs_node *kn)
/**
* cgroup_kn_lock_live - locking helper for cgroup kernfs methods
* @kn: the kernfs_node being serviced
* @drain_offline: perform offline draining on the cgroup
*
* This helper is to be used by a cgroup kernfs method currently servicing
* @kn. It breaks the active protection, performs cgroup locking and
* verifies that the associated cgroup is alive. Returns the cgroup if
* alive; otherwise, %NULL. A successful return should be undone by a
* matching cgroup_kn_unlock() invocation.
* matching cgroup_kn_unlock() invocation. If @drain_offline is %true, the
* cgroup is drained of offlining csses before return.
*
* Any cgroup kernfs method implementation which requires locking the
* associated cgroup should use this helper. It avoids nesting cgroup
* locking under kernfs active protection and allows all kernfs operations
* including self-removal.
*/
static struct cgroup *cgroup_kn_lock_live(struct kernfs_node *kn)
static struct cgroup *cgroup_kn_lock_live(struct kernfs_node *kn,
bool drain_offline)
{
struct cgroup *cgrp;
@ -1422,7 +1426,10 @@ static struct cgroup *cgroup_kn_lock_live(struct kernfs_node *kn)
return NULL;
kernfs_break_active_protection(kn);
mutex_lock(&cgroup_mutex);
if (drain_offline)
cgroup_lock_and_drain_offline(cgrp);
else
mutex_lock(&cgroup_mutex);
if (!cgroup_is_dead(cgrp))
return cgrp;
@ -2761,7 +2768,7 @@ static ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf,
if (kstrtoint(strstrip(buf), 0, &pid) || pid < 0)
return -EINVAL;
cgrp = cgroup_kn_lock_live(of->kn);
cgrp = cgroup_kn_lock_live(of->kn, false);
if (!cgrp)
return -ENODEV;
@ -2859,7 +2866,7 @@ static ssize_t cgroup_release_agent_write(struct kernfs_open_file *of,
BUILD_BUG_ON(sizeof(cgrp->root->release_agent_path) < PATH_MAX);
cgrp = cgroup_kn_lock_live(of->kn);
cgrp = cgroup_kn_lock_live(of->kn, false);
if (!cgrp)
return -ENODEV;
spin_lock(&release_agent_path_lock);
@ -2984,27 +2991,23 @@ out_finish:
}
/**
* cgroup_drain_offline - wait for previously offlined csses to go away
* cgroup_lock_and_drain_offline - lock cgroup_mutex and drain offlined csses
* @cgrp: root of the target subtree
*
* Because css offlining is asynchronous, userland may try to re-enable a
* controller while the previous css is still around. This function drains
* the previous css instances of @cgrp's subtree.
*
* Must be called with cgroup_mutex held. Returns %false if there were no
* dying css instances. Returns %true if there were one or more and this
* function waited. On %true return, cgroup_mutex has been dropped and
* re-acquired inbetween which anything could have happened. The caller
* typically would have to start over.
* controller while the previous css is still around. This function grabs
* cgroup_mutex and drains the previous css instances of @cgrp's subtree.
*/
static bool cgroup_drain_offline(struct cgroup *cgrp)
static void cgroup_lock_and_drain_offline(struct cgroup *cgrp)
__acquires(&cgroup_mutex)
{
struct cgroup *dsct;
struct cgroup_subsys_state *d_css;
struct cgroup_subsys *ss;
int ssid;
lockdep_assert_held(&cgroup_mutex);
restart:
mutex_lock(&cgroup_mutex);
cgroup_for_each_live_descendant_post(dsct, d_css, cgrp) {
for_each_subsys(ss, ssid) {
@ -3021,14 +3024,11 @@ static bool cgroup_drain_offline(struct cgroup *cgrp)
mutex_unlock(&cgroup_mutex);
schedule();
finish_wait(&dsct->offline_waitq, &wait);
mutex_lock(&cgroup_mutex);
cgroup_put(dsct);
return true;
goto restart;
}
}
return false;
}
/**
@ -3111,6 +3111,8 @@ static int cgroup_apply_control_enable(struct cgroup *cgrp)
for_each_subsys(ss, ssid) {
struct cgroup_subsys_state *css = cgroup_css(dsct, ss);
WARN_ON_ONCE(css && percpu_ref_is_dying(&css->refcnt));
if (!(cgroup_ss_mask(dsct) & (1 << ss->id)))
continue;
@ -3155,6 +3157,8 @@ static void cgroup_apply_control_disable(struct cgroup *cgrp)
for_each_subsys(ss, ssid) {
struct cgroup_subsys_state *css = cgroup_css(dsct, ss);
WARN_ON_ONCE(css && percpu_ref_is_dying(&css->refcnt));
if (!css)
continue;
@ -3264,7 +3268,7 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
return -EINVAL;
}
cgrp = cgroup_kn_lock_live(of->kn);
cgrp = cgroup_kn_lock_live(of->kn, true);
if (!cgrp)
return -ENODEV;
@ -3309,11 +3313,6 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
goto out_unlock;
}
if (cgroup_drain_offline(cgrp)) {
cgroup_kn_unlock(of->kn);
return restart_syscall();
}
/* save and update control masks and prepare csses */
cgroup_save_control(cgrp);
@ -5140,7 +5139,7 @@ static int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name,
if (strchr(name, '\n'))
return -EINVAL;
parent = cgroup_kn_lock_live(parent_kn);
parent = cgroup_kn_lock_live(parent_kn, false);
if (!parent)
return -ENODEV;
@ -5339,7 +5338,7 @@ static int cgroup_rmdir(struct kernfs_node *kn)
struct cgroup *cgrp;
int ret = 0;
cgrp = cgroup_kn_lock_live(kn);
cgrp = cgroup_kn_lock_live(kn, false);
if (!cgrp)
return 0;