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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
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38
include/linux/cgroup.h
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@@ -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);