cgroup: make cgroup_css() take cgroup_subsys * instead and allow NULL subsys

cgroup_css() is no longer used in hot paths.  Make it take struct
cgroup_subsys * and allow the users to specify NULL subsys to obtain
the dummy_css.  This removes open-coded NULL subsystem testing in a
couple users and generally simplifies the code.

After this patch, css_from_dir() also allows NULL @ss and returns the
matching dummy_css.  This behavior change doesn't affect its only user
- perf.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

1 files changed, 43 insertions, 47 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 921b1387c944..7516668d8325 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -226,19 +226,22 @@ static int cgroup_addrm_files(struct cgroup *cgrp, struct cftype cfts[],
 /**
  * cgroup_css - obtain a cgroup's css for the specified subsystem
  * @cgrp: the cgroup of interest
- * @subsys_id: the subsystem of interest
+ * @ss: the subsystem of interest (%NULL returns the dummy_css)
  *
- * Return @cgrp's css (cgroup_subsys_state) associated with @subsys_id.
- * This function must be called either under cgroup_mutex or
- * rcu_read_lock() and the caller is responsible for pinning the returned
- * css if it wants to keep accessing it outside the said locks.  This
- * function may return %NULL if @cgrp doesn't have @subsys_id enabled.
+ * Return @cgrp's css (cgroup_subsys_state) associated with @ss.  This
+ * function must be called either under cgroup_mutex or rcu_read_lock() and
+ * the caller is responsible for pinning the returned css if it wants to
+ * keep accessing it outside the said locks.  This function may return
+ * %NULL if @cgrp doesn't have @subsys_id enabled.
  */
 static struct cgroup_subsys_state *cgroup_css(struct cgroup *cgrp,
-                                              int subsys_id)
+                                              struct cgroup_subsys *ss)
 {
-        return rcu_dereference_check(cgrp->subsys[subsys_id],
-                                     lockdep_is_held(&cgroup_mutex));
+        if (ss)
+                return rcu_dereference_check(cgrp->subsys[ss->subsys_id],
+                                             lockdep_is_held(&cgroup_mutex));
+        else
+                return &cgrp->dummy_css;
 }
 
 /* convenient tests for these bits */
@@ -580,7 +583,7 @@ static struct css_set *find_existing_css_set(struct css_set *old_cset,
                         /* Subsystem is in this hierarchy. So we want
                          * the subsystem state from the new
                          * cgroup */
-                        template[i] = cgroup_css(cgrp, i);
+                        template[i] = cgroup_css(cgrp, ss);
                 } else {
                         /* Subsystem is not in this hierarchy, so we
                          * don't want to change the subsystem state */
@@ -1062,30 +1065,30 @@ static int rebind_subsystems(struct cgroupfs_root *root,
 
                 if (bit & added_mask) {
                         /* We're binding this subsystem to this hierarchy */
-                        BUG_ON(cgroup_css(cgrp, i));
-                        BUG_ON(!cgroup_css(cgroup_dummy_top, i));
-                        BUG_ON(cgroup_css(cgroup_dummy_top, i)->cgroup != cgroup_dummy_top);
+                        BUG_ON(cgroup_css(cgrp, ss));
+                        BUG_ON(!cgroup_css(cgroup_dummy_top, ss));
+                        BUG_ON(cgroup_css(cgroup_dummy_top, ss)->cgroup != cgroup_dummy_top);
 
                         rcu_assign_pointer(cgrp->subsys[i],
-                                           cgroup_css(cgroup_dummy_top, i));
-                        cgroup_css(cgrp, i)->cgroup = cgrp;
+                                           cgroup_css(cgroup_dummy_top, ss));
+                        cgroup_css(cgrp, ss)->cgroup = cgrp;
 
                         list_move(&ss->sibling, &root->subsys_list);
                         ss->root = root;
                         if (ss->bind)
-                                ss->bind(cgroup_css(cgrp, i));
+                                ss->bind(cgroup_css(cgrp, ss));
 
                         /* refcount was already taken, and we're keeping it */
                         root->subsys_mask |= bit;
                 } else if (bit & removed_mask) {
                         /* We're removing this subsystem */
-                        BUG_ON(cgroup_css(cgrp, i) != cgroup_css(cgroup_dummy_top, i));
-                        BUG_ON(cgroup_css(cgrp, i)->cgroup != cgrp);
+                        BUG_ON(cgroup_css(cgrp, ss) != cgroup_css(cgroup_dummy_top, ss));
+                        BUG_ON(cgroup_css(cgrp, ss)->cgroup != cgrp);
 
                         if (ss->bind)
-                                ss->bind(cgroup_css(cgroup_dummy_top, i));
+                                ss->bind(cgroup_css(cgroup_dummy_top, ss));
 
-                        cgroup_css(cgroup_dummy_top, i)->cgroup = cgroup_dummy_top;
+                        cgroup_css(cgroup_dummy_top, ss)->cgroup = cgroup_dummy_top;
                         RCU_INIT_POINTER(cgrp->subsys[i], NULL);
 
                         cgroup_subsys[i]->root = &cgroup_dummy_root;
@@ -1930,7 +1933,7 @@ EXPORT_SYMBOL_GPL(cgroup_taskset_next);
 struct cgroup_subsys_state *cgroup_taskset_cur_css(struct cgroup_taskset *tset,
                                                    int subsys_id)
 {
-        return cgroup_css(tset->cur_cgrp, subsys_id);
+        return cgroup_css(tset->cur_cgrp, cgroup_subsys[subsys_id]);
 }
 EXPORT_SYMBOL_GPL(cgroup_taskset_cur_css);
 
@@ -2071,7 +2074,7 @@ static int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk,
          * step 1: check that we can legitimately attach to the cgroup.
          */
         for_each_root_subsys(root, ss) {
-                struct cgroup_subsys_state *css = cgroup_css(cgrp, ss->subsys_id);
+                struct cgroup_subsys_state *css = cgroup_css(cgrp, ss);
 
                 if (ss->can_attach) {
                         retval = ss->can_attach(css, &tset);
@@ -2113,7 +2116,7 @@ static int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk,
          * step 4: do subsystem attach callbacks.
          */
         for_each_root_subsys(root, ss) {
-                struct cgroup_subsys_state *css = cgroup_css(cgrp, ss->subsys_id);
+                struct cgroup_subsys_state *css = cgroup_css(cgrp, ss);
 
                 if (ss->attach)
                         ss->attach(css, &tset);
@@ -2135,7 +2138,7 @@ out_put_css_set_refs:
 out_cancel_attach:
         if (retval) {
                 for_each_root_subsys(root, ss) {
-                        struct cgroup_subsys_state *css = cgroup_css(cgrp, ss->subsys_id);
+                        struct cgroup_subsys_state *css = cgroup_css(cgrp, ss);
 
                         if (ss == failed_ss)
                                 break;
@@ -2481,13 +2484,9 @@ static int cgroup_file_open(struct inode *inode, struct file *file)
          * @css stays alive for all file operations.
          */
         rcu_read_lock();
-        if (cft->ss) {
-                css = cgroup_css(cgrp, cft->ss->subsys_id);
-                if (!css_tryget(css))
-                        css = NULL;
-        } else {
-                css = &cgrp->dummy_css;
-        }
+        css = cgroup_css(cgrp, cft->ss);
+        if (cft->ss && !css_tryget(css))
+                css = NULL;
         rcu_read_unlock();
 
         if (!css)
@@ -2878,7 +2877,7 @@ static int cgroup_cfts_commit(struct cftype *cfts, bool is_add)
 
         /* add/rm files for all cgroups created before */
         rcu_read_lock();
-        css_for_each_descendant_pre(css, cgroup_css(root, ss->subsys_id)) {
+        css_for_each_descendant_pre(css, cgroup_css(root, ss)) {
                 struct cgroup *cgrp = css->cgroup;
 
                 if (cgroup_is_dead(cgrp))
@@ -3082,10 +3081,7 @@ css_next_child(struct cgroup_subsys_state *pos_css,
         if (&next->sibling == &cgrp->children)
                 return NULL;
 
-        if (parent_css->ss)
-                return cgroup_css(next, parent_css->ss->subsys_id);
-        else
-                return &next->dummy_css;
+        return cgroup_css(next, parent_css->ss);
 }
 EXPORT_SYMBOL_GPL(css_next_child);
 
@@ -4110,7 +4106,7 @@ static int cgroup_write_event_control(struct cgroup_subsys_state *dummy_css,
         rcu_read_lock();
 
         ret = -EINVAL;
-        event->css = cgroup_css(cgrp, event->cft->ss->subsys_id);
+        event->css = cgroup_css(cgrp, event->cft->ss);
         if (event->css)
                 ret = 0;
 
@@ -4266,7 +4262,7 @@ static int cgroup_populate_dir(struct cgroup *cgrp, unsigned long subsys_mask)
 
         /* This cgroup is ready now */
         for_each_root_subsys(cgrp->root, ss) {
-                struct cgroup_subsys_state *css = cgroup_css(cgrp, ss->subsys_id);
+                struct cgroup_subsys_state *css = cgroup_css(cgrp, ss);
                 struct css_id *id = rcu_dereference_protected(css->id, true);
 
                 /*
@@ -4349,11 +4345,11 @@ static void init_css(struct cgroup_subsys_state *css, struct cgroup_subsys *ss,
         css->id = NULL;
 
         if (cgrp->parent)
-                css->parent = cgroup_css(cgrp->parent, ss->subsys_id);
+                css->parent = cgroup_css(cgrp->parent, ss);
         else
                 css->flags |= CSS_ROOT;
 
-        BUG_ON(cgroup_css(cgrp, ss->subsys_id));
+        BUG_ON(cgroup_css(cgrp, ss));
 }
 
 /* invoke ->css_online() on a new CSS and mark it online if successful */
@@ -4466,7 +4462,7 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
         for_each_root_subsys(root, ss) {
                 struct cgroup_subsys_state *css;
 
-                css = ss->css_alloc(cgroup_css(parent, ss->subsys_id));
+                css = ss->css_alloc(cgroup_css(parent, ss));
                 if (IS_ERR(css)) {
                         err = PTR_ERR(css);
                         goto err_free_all;
@@ -4712,7 +4708,7 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
          * percpu refs of all css's are confirmed to be killed.
          */
         for_each_root_subsys(cgrp->root, ss)
-                kill_css(cgroup_css(cgrp, ss->subsys_id));
+                kill_css(cgroup_css(cgrp, ss));
 
         /*
          * Mark @cgrp dead.  This prevents further task migration and child
@@ -4839,7 +4835,7 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss)
         /* Create the top cgroup state for this subsystem */
         list_add(&ss->sibling, &cgroup_dummy_root.subsys_list);
         ss->root = &cgroup_dummy_root;
-        css = ss->css_alloc(cgroup_css(cgroup_dummy_top, ss->subsys_id));
+        css = ss->css_alloc(cgroup_css(cgroup_dummy_top, ss));
         /* We don't handle early failures gracefully */
         BUG_ON(IS_ERR(css));
         init_css(css, ss, cgroup_dummy_top);
@@ -4918,7 +4914,7 @@ int __init_or_module cgroup_load_subsys(struct cgroup_subsys *ss)
          * struct, so this can happen first (i.e. before the dummy root
          * attachment).
          */
-        css = ss->css_alloc(cgroup_css(cgroup_dummy_top, ss->subsys_id));
+        css = ss->css_alloc(cgroup_css(cgroup_dummy_top, ss));
         if (IS_ERR(css)) {
                 /* failure case - need to deassign the cgroup_subsys[] slot. */
                 cgroup_subsys[ss->subsys_id] = NULL;
@@ -5000,7 +4996,7 @@ void cgroup_unload_subsys(struct cgroup_subsys *ss)
 
         mutex_lock(&cgroup_mutex);
 
-        offline_css(cgroup_css(cgroup_dummy_top, ss->subsys_id));
+        offline_css(cgroup_css(cgroup_dummy_top, ss));
 
         if (ss->use_id)
                 idr_destroy(&ss->idr);
@@ -5034,7 +5030,7 @@ void cgroup_unload_subsys(struct cgroup_subsys *ss)
          * the cgrp->subsys pointer to find their state. note that this
          * also takes care of freeing the css_id.
          */
-        ss->css_free(cgroup_css(cgroup_dummy_top, ss->subsys_id));
+        ss->css_free(cgroup_css(cgroup_dummy_top, ss));
         RCU_INIT_POINTER(cgroup_dummy_top->subsys[ss->subsys_id], NULL);
 
         mutex_unlock(&cgroup_mutex);
@@ -5721,7 +5717,7 @@ struct cgroup_subsys_state *css_from_dir(struct dentry *dentry,
                 return ERR_PTR(-EBADF);
 
         cgrp = __d_cgrp(dentry);
-        return cgroup_css(cgrp, ss->subsys_id) ?: ERR_PTR(-ENOENT);
+        return cgroup_css(cgrp, ss) ?: ERR_PTR(-ENOENT);
 }
 
 /**