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cgroups: add css_tryget()

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Add css_tryget(), that obtains a counted reference on a CSS.  It is used
in situations where the caller has a "weak" reference to the CSS, i.e.
one that does not protect the cgroup from removal via a reference count,
but would instead be cleaned up by a destroy() callback.

css_tryget() will return true on success, or false if the cgroup is being
removed.

This is similar to Kamezawa Hiroyuki's patch from a week or two ago, but
with the difference that in the event of css_tryget() racing with a
cgroup_rmdir(), css_tryget() will only return false if the cgroup really
does get removed.

This implementation is done by biasing css->refcnt, so that a refcnt of 1
means "releasable" and 0 means "released or releasing".  In the event of a
race, css_tryget() distinguishes between "released" and "releasing" by
checking for the CSS_REMOVED flag in css->flags.

Signed-off-by: Paul Menage <menage@google.com>
Tested-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Paul Menage 2009-01-07 18:08:38 -08:00 committed by Linus Torvalds
parent 2cb378c862
commit e7c5ec9193
2 changed files with 88 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

38
include/linux/cgroup.h
View File

@ -52,9 +52,9 @@ struct cgroup_subsys_state {
* hierarchy structure */
struct cgroup *cgroup;
/* State maintained by the cgroup system to allow
* subsystems to be "busy". Should be accessed via css_get()
* and css_put() */
/* State maintained by the cgroup system to allow subsystems
* to be "busy". Should be accessed via css_get(),
* css_tryget() and and css_put(). */
atomic_t refcnt;
@ -64,11 +64,14 @@ struct cgroup_subsys_state {
/* bits in struct cgroup_subsys_state flags field */
enum {
CSS_ROOT, /* This CSS is the root of the subsystem */
CSS_REMOVED, /* This CSS is dead */
};
/*
* Call css_get() to hold a reference on the cgroup;
*
* Call css_get() to hold a reference on the css; it can be used
* for a reference obtained via:
* - an existing ref-counted reference to the css
* - task->cgroups for a locked task
*/
static inline void css_get(struct cgroup_subsys_state *css)
@ -77,9 +80,32 @@ static inline void css_get(struct cgroup_subsys_state *css)
if (!test_bit(CSS_ROOT, &css->flags))
atomic_inc(&css->refcnt);
}
static inline bool css_is_removed(struct cgroup_subsys_state *css)
{
return test_bit(CSS_REMOVED, &css->flags);
}
/*
* Call css_tryget() to take a reference on a css if your existing
* (known-valid) reference isn't already ref-counted. Returns false if
* the css has been destroyed.
*/
static inline bool css_tryget(struct cgroup_subsys_state *css)
{
if (test_bit(CSS_ROOT, &css->flags))
return true;
while (!atomic_inc_not_zero(&css->refcnt)) {
if (test_bit(CSS_REMOVED, &css->flags))
return false;
}
return true;
}
/*
* css_put() should be called to release a reference taken by
* css_get()
* css_get() or css_tryget()
*/
extern void __css_put(struct cgroup_subsys_state *css);

61
kernel/cgroup.c
View File

@ -2333,7 +2333,7 @@ static void init_cgroup_css(struct cgroup_subsys_state *css,
struct cgroup *cgrp)
{
css->cgroup = cgrp;
atomic_set(&css->refcnt, 0);
atomic_set(&css->refcnt, 1);
css->flags = 0;
if (cgrp == dummytop)
set_bit(CSS_ROOT, &css->flags);
@ -2465,7 +2465,7 @@ static int cgroup_has_css_refs(struct cgroup *cgrp)
{
/* Check the reference count on each subsystem. Since we
* already established that there are no tasks in the
* cgroup, if the css refcount is also 0, then there should
* cgroup, if the css refcount is also 1, then there should
* be no outstanding references, so the subsystem is safe to
* destroy. We scan across all subsystems rather than using
* the per-hierarchy linked list of mounted subsystems since
@ -2486,12 +2486,62 @@ static int cgroup_has_css_refs(struct cgroup *cgrp)
* matter, since it can only happen if the cgroup
* has been deleted and hence no longer needs the
* release agent to be called anyway. */
if (css && atomic_read(&css->refcnt))
if (css && (atomic_read(&css->refcnt) > 1))
return 1;
}
return 0;
}
/*
* Atomically mark all (or else none) of the cgroup's CSS objects as
* CSS_REMOVED. Return true on success, or false if the cgroup has
* busy subsystems. Call with cgroup_mutex held
*/
static int cgroup_clear_css_refs(struct cgroup *cgrp)
{
struct cgroup_subsys *ss;
unsigned long flags;
bool failed = false;
local_irq_save(flags);
for_each_subsys(cgrp->root, ss) {
struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
int refcnt;
do {
/* We can only remove a CSS with a refcnt==1 */
refcnt = atomic_read(&css->refcnt);
if (refcnt > 1) {
failed = true;
goto done;
}
BUG_ON(!refcnt);
/*
* Drop the refcnt to 0 while we check other
* subsystems. This will cause any racing
* css_tryget() to spin until we set the
* CSS_REMOVED bits or abort
*/
} while (atomic_cmpxchg(&css->refcnt, refcnt, 0) != refcnt);
}
done:
for_each_subsys(cgrp->root, ss) {
struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
if (failed) {
/*
* Restore old refcnt if we previously managed
* to clear it from 1 to 0
*/
if (!atomic_read(&css->refcnt))
atomic_set(&css->refcnt, 1);
} else {
/* Commit the fact that the CSS is removed */
set_bit(CSS_REMOVED, &css->flags);
}
}
local_irq_restore(flags);
return !failed;
}
static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
{
struct cgroup *cgrp = dentry->d_fsdata;
@ -2522,7 +2572,7 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
if (atomic_read(&cgrp->count)
|| !list_empty(&cgrp->children)
|| cgroup_has_css_refs(cgrp)) {
|| !cgroup_clear_css_refs(cgrp)) {
mutex_unlock(&cgroup_mutex);
return -EBUSY;
}
@ -3078,7 +3128,8 @@ void __css_put(struct cgroup_subsys_state *css)
{
struct cgroup *cgrp = css->cgroup;
rcu_read_lock();
if (atomic_dec_and_test(&css->refcnt) && notify_on_release(cgrp)) {
if ((atomic_dec_return(&css->refcnt) == 1) &&
notify_on_release(cgrp)) {
set_bit(CGRP_RELEASABLE, &cgrp->flags);
check_for_release(cgrp);
}