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    Subject[RFC][PATCH 3/3] CGroups: Add css_tryget()
    This patch adds 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>

    ---
    include/linux/cgroup.h | 33 ++++++++++++++++++++++-----
    kernel/cgroup.c | 58 ++++++++++++++++++++++++++++++++++++++++++++-----
    2 files changed, 80 insertions(+), 11 deletions(-)

    Index: hierarchy_lock-mmotm-2008-12-09/include/linux/cgroup.h
    ===================================================================
    --- hierarchy_lock-mmotm-2008-12-09.orig/include/linux/cgroup.h
    +++ hierarchy_lock-mmotm-2008-12-09/include/linux/cgroup.h
    @@ -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 +80,27 @@ static inline void css_get(struct cgroup
    if (!test_bit(CSS_ROOT, &css->flags))
    atomic_inc(&css->refcnt);
    }
    +
    +/*
    + * 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);
    Index: hierarchy_lock-mmotm-2008-12-09/kernel/cgroup.c
    ===================================================================
    --- hierarchy_lock-mmotm-2008-12-09.orig/kernel/cgroup.c
    +++ hierarchy_lock-mmotm-2008-12-09/kernel/cgroup.c
    @@ -2321,7 +2321,7 @@ static void init_cgroup_css(struct cgrou
    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);
    @@ -2453,7 +2453,7 @@ static int cgroup_has_css_refs(struct cg
    {
    /* 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
    @@ -2474,12 +2474,59 @@ static int cgroup_has_css_refs(struct cg
    * 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 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 necessary */
    + 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;
    @@ -2510,7 +2557,7 @@ static int cgroup_rmdir(struct inode *un

    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;
    }
    @@ -3065,7 +3112,8 @@ void __css_put(struct cgroup_subsys_stat
    {
    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);
    }
    --


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