cgroups: add css_tryget()
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>
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2 changed files with 88 additions and 11 deletions
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@ -52,9 +52,9 @@ struct cgroup_subsys_state {
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* hierarchy structure */
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struct cgroup *cgroup;
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/* State maintained by the cgroup system to allow
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* subsystems to be "busy". Should be accessed via css_get()
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* and css_put() */
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/* State maintained by the cgroup system to allow subsystems
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* to be "busy". Should be accessed via css_get(),
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* css_tryget() and and css_put(). */
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atomic_t refcnt;
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@ -64,11 +64,14 @@ struct cgroup_subsys_state {
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/* bits in struct cgroup_subsys_state flags field */
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enum {
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CSS_ROOT, /* This CSS is the root of the subsystem */
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CSS_REMOVED, /* This CSS is dead */
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};
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/*
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* Call css_get() to hold a reference on the cgroup;
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*
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* Call css_get() to hold a reference on the css; it can be used
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* for a reference obtained via:
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* - an existing ref-counted reference to the css
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* - task->cgroups for a locked task
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*/
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static inline void css_get(struct cgroup_subsys_state *css)
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@ -77,9 +80,32 @@ static inline void css_get(struct cgroup_subsys_state *css)
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if (!test_bit(CSS_ROOT, &css->flags))
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atomic_inc(&css->refcnt);
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}
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static inline bool css_is_removed(struct cgroup_subsys_state *css)
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{
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return test_bit(CSS_REMOVED, &css->flags);
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}
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/*
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* Call css_tryget() to take a reference on a css if your existing
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* (known-valid) reference isn't already ref-counted. Returns false if
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* the css has been destroyed.
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*/
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static inline bool css_tryget(struct cgroup_subsys_state *css)
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{
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if (test_bit(CSS_ROOT, &css->flags))
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return true;
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while (!atomic_inc_not_zero(&css->refcnt)) {
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if (test_bit(CSS_REMOVED, &css->flags))
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return false;
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}
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return true;
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}
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/*
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* css_put() should be called to release a reference taken by
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* css_get()
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* css_get() or css_tryget()
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*/
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extern void __css_put(struct cgroup_subsys_state *css);
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@ -2333,7 +2333,7 @@ static void init_cgroup_css(struct cgroup_subsys_state *css,
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struct cgroup *cgrp)
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{
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css->cgroup = cgrp;
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atomic_set(&css->refcnt, 0);
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atomic_set(&css->refcnt, 1);
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css->flags = 0;
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if (cgrp == dummytop)
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set_bit(CSS_ROOT, &css->flags);
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@ -2465,7 +2465,7 @@ static int cgroup_has_css_refs(struct cgroup *cgrp)
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{
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/* Check the reference count on each subsystem. Since we
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* already established that there are no tasks in the
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* cgroup, if the css refcount is also 0, then there should
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* cgroup, if the css refcount is also 1, then there should
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* be no outstanding references, so the subsystem is safe to
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* destroy. We scan across all subsystems rather than using
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* the per-hierarchy linked list of mounted subsystems since
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@ -2486,12 +2486,62 @@ static int cgroup_has_css_refs(struct cgroup *cgrp)
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* matter, since it can only happen if the cgroup
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* has been deleted and hence no longer needs the
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* release agent to be called anyway. */
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if (css && atomic_read(&css->refcnt))
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if (css && (atomic_read(&css->refcnt) > 1))
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return 1;
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}
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return 0;
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}
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/*
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* Atomically mark all (or else none) of the cgroup's CSS objects as
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* CSS_REMOVED. Return true on success, or false if the cgroup has
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* busy subsystems. Call with cgroup_mutex held
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*/
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static int cgroup_clear_css_refs(struct cgroup *cgrp)
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{
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struct cgroup_subsys *ss;
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unsigned long flags;
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bool failed = false;
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local_irq_save(flags);
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for_each_subsys(cgrp->root, ss) {
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struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
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int refcnt;
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do {
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/* We can only remove a CSS with a refcnt==1 */
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refcnt = atomic_read(&css->refcnt);
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if (refcnt > 1) {
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failed = true;
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goto done;
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}
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BUG_ON(!refcnt);
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/*
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* Drop the refcnt to 0 while we check other
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* subsystems. This will cause any racing
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* css_tryget() to spin until we set the
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* CSS_REMOVED bits or abort
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*/
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} while (atomic_cmpxchg(&css->refcnt, refcnt, 0) != refcnt);
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}
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done:
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for_each_subsys(cgrp->root, ss) {
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struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
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if (failed) {
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/*
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* Restore old refcnt if we previously managed
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* to clear it from 1 to 0
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*/
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if (!atomic_read(&css->refcnt))
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atomic_set(&css->refcnt, 1);
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} else {
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/* Commit the fact that the CSS is removed */
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set_bit(CSS_REMOVED, &css->flags);
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}
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}
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local_irq_restore(flags);
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return !failed;
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}
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static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
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{
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struct cgroup *cgrp = dentry->d_fsdata;
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@ -2522,7 +2572,7 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
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if (atomic_read(&cgrp->count)
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|| !list_empty(&cgrp->children)
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|| cgroup_has_css_refs(cgrp)) {
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|| !cgroup_clear_css_refs(cgrp)) {
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mutex_unlock(&cgroup_mutex);
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return -EBUSY;
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}
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@ -3078,7 +3128,8 @@ void __css_put(struct cgroup_subsys_state *css)
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{
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struct cgroup *cgrp = css->cgroup;
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rcu_read_lock();
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if (atomic_dec_and_test(&css->refcnt) && notify_on_release(cgrp)) {
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if ((atomic_dec_return(&css->refcnt) == 1) &&
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notify_on_release(cgrp)) {
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set_bit(CGRP_RELEASABLE, &cgrp->flags);
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check_for_release(cgrp);
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}
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