diff options
Diffstat (limited to 'kernel')
| -rw-r--r-- | kernel/cgroup.c | 119 |
1 files changed, 71 insertions, 48 deletions
diff --git a/kernel/cgroup.c b/kernel/cgroup.c index 21bba772235..2eade518660 100644 --- a/kernel/cgroup.c +++ b/kernel/cgroup.c @@ -63,6 +63,9 @@ #include <linux/atomic.h> +/* css deactivation bias, makes css->refcnt negative to deny new trygets */ +#define CSS_DEACT_BIAS INT_MIN + /* * cgroup_mutex is the master lock. Any modification to cgroup or its * hierarchy must be performed while holding it. @@ -251,6 +254,14 @@ int cgroup_lock_is_held(void) EXPORT_SYMBOL_GPL(cgroup_lock_is_held); +/* the current nr of refs, always >= 0 whether @css is deactivated or not */ +static int css_refcnt(struct cgroup_subsys_state *css) +{ + int v = atomic_read(&css->refcnt); + + return v >= 0 ? v : v - CSS_DEACT_BIAS; +} + /* convenient tests for these bits */ inline int cgroup_is_removed(const struct cgroup *cgrp) { @@ -4006,18 +4017,19 @@ static int cgroup_mkdir(struct inode *dir, struct dentry *dentry, umode_t mode) return cgroup_create(c_parent, dentry, mode | S_IFDIR); } +/* + * 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 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 we can + * be called via check_for_release() with no synchronization other than + * RCU, and the subsystem linked list isn't RCU-safe. + */ 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 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 - * we can be called via check_for_release() with no - * synchronization other than RCU, and the subsystem linked - * list isn't RCU-safe */ int i; + /* * We won't need to lock the subsys array, because the subsystems * we're concerned about aren't going anywhere since our cgroup root @@ -4026,17 +4038,21 @@ static int cgroup_has_css_refs(struct cgroup *cgrp) for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) { struct cgroup_subsys *ss = subsys[i]; struct cgroup_subsys_state *css; + /* Skip subsystems not present or not in this hierarchy */ if (ss == NULL || ss->root != cgrp->root) continue; + css = cgrp->subsys[ss->subsys_id]; - /* When called from check_for_release() it's possible + /* + * When called from check_for_release() it's possible * that by this point the cgroup has been removed * and the css deleted. But a false-positive doesn't * 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) > 1)) + * release agent to be called anyway. + */ + if (css && css_refcnt(css) > 1) return 1; } return 0; @@ -4053,44 +4069,37 @@ static int cgroup_clear_css_refs(struct cgroup *cgrp) struct cgroup_subsys *ss; unsigned long flags; bool failed = false; + local_irq_save(flags); + + /* + * Block new css_tryget() by deactivating refcnt. If all refcnts + * were 1 at the moment of deactivation, we succeeded. + */ for_each_subsys(cgrp->root, ss) { struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id]; - int refcnt; - while (1) { - /* 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 - */ - if (atomic_cmpxchg(&css->refcnt, refcnt, 0) == refcnt) - break; - cpu_relax(); - } + + WARN_ON(atomic_read(&css->refcnt) < 0); + atomic_add(CSS_DEACT_BIAS, &css->refcnt); + failed |= css_refcnt(css) != 1; } - done: + + /* + * If succeeded, set REMOVED and put all the base refs; otherwise, + * restore refcnts to positive values. Either way, all in-progress + * css_tryget() will be released. + */ 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 */ + + if (!failed) { set_bit(CSS_REMOVED, &css->flags); + css_put(css); + } else { + atomic_sub(CSS_DEACT_BIAS, &css->refcnt); } } + local_irq_restore(flags); return !failed; } @@ -4887,13 +4896,28 @@ static void check_for_release(struct cgroup *cgrp) } /* Caller must verify that the css is not for root cgroup */ -void __css_put(struct cgroup_subsys_state *css, int count) +bool __css_tryget(struct cgroup_subsys_state *css) +{ + do { + int v = css_refcnt(css); + + if (atomic_cmpxchg(&css->refcnt, v, v + 1) == v) + return true; + cpu_relax(); + } while (!test_bit(CSS_REMOVED, &css->flags)); + + return false; +} +EXPORT_SYMBOL_GPL(__css_tryget); + +/* Caller must verify that the css is not for root cgroup */ +void __css_put(struct cgroup_subsys_state *css) { struct cgroup *cgrp = css->cgroup; - int val; + rcu_read_lock(); - val = atomic_sub_return(count, &css->refcnt); - if (val == 1) { + atomic_dec(&css->refcnt); + if (css_refcnt(css) == 1) { if (notify_on_release(cgrp)) { set_bit(CGRP_RELEASABLE, &cgrp->flags); check_for_release(cgrp); @@ -4901,7 +4925,6 @@ void __css_put(struct cgroup_subsys_state *css, int count) cgroup_wakeup_rmdir_waiter(cgrp); } rcu_read_unlock(); - WARN_ON_ONCE(val < 1); } EXPORT_SYMBOL_GPL(__css_put); @@ -5020,7 +5043,7 @@ unsigned short css_id(struct cgroup_subsys_state *css) * on this or this is under rcu_read_lock(). Once css->id is allocated, * it's unchanged until freed. */ - cssid = rcu_dereference_check(css->id, atomic_read(&css->refcnt)); + cssid = rcu_dereference_check(css->id, css_refcnt(css)); if (cssid) return cssid->id; @@ -5032,7 +5055,7 @@ unsigned short css_depth(struct cgroup_subsys_state *css) { struct css_id *cssid; - cssid = rcu_dereference_check(css->id, atomic_read(&css->refcnt)); + cssid = rcu_dereference_check(css->id, css_refcnt(css)); if (cssid) return cssid->depth; |