cgroup: make cgroup_task_iter remember the cgroup being iterated

Currently all cgroup_task_iter functions require @cgrp to be passed
in, which is superflous and increases chance of usage error.  Make
cgroup_task_iter remember the cgroup being iterated and drop @cgrp
argument from next and end functions.

This patch doesn't introduce any behavior differences.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Matt Helsley <matthltc@us.ibm.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>

4 files changed, 27 insertions, 29 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index ea439794bd9b..0287fccd0f54 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -891,14 +891,14 @@ css_next_descendant_post(struct cgroup_subsys_state *pos,
 
 /* A cgroup_task_iter should be treated as an opaque object */
 struct cgroup_task_iter {
+        struct cgroup                   *origin_cgrp;
         struct list_head                *cset_link;
         struct list_head                *task;
 };
 
 void cgroup_task_iter_start(struct cgroup *cgrp, struct cgroup_task_iter *it);
-struct task_struct *cgroup_task_iter_next(struct cgroup *cgrp,
-                                          struct cgroup_task_iter *it);
-void cgroup_task_iter_end(struct cgroup *cgrp, struct cgroup_task_iter *it);
+struct task_struct *cgroup_task_iter_next(struct cgroup_task_iter *it);
+void cgroup_task_iter_end(struct cgroup_task_iter *it);
 int cgroup_scan_tasks(struct cgroup_scanner *scan);
 int cgroup_attach_task_all(struct task_struct *from, struct task_struct *);
 int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from);
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 15c93f9c9e57..abc62ea1303c 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3205,13 +3205,11 @@ EXPORT_SYMBOL_GPL(css_next_descendant_post);
 
 /**
  * cgroup_advance_task_iter - advance a task itererator to the next css_set
- * @cgrp: the cgroup to walk tasks of
  * @it: the iterator to advance
  *
  * Advance @it to the next css_set to walk.
  */
-static void cgroup_advance_task_iter(struct cgroup *cgrp,
-                                     struct cgroup_task_iter *it)
+static void cgroup_advance_task_iter(struct cgroup_task_iter *it)
 {
         struct list_head *l = it->cset_link;
         struct cgrp_cset_link *link;
@@ -3220,7 +3218,7 @@ static void cgroup_advance_task_iter(struct cgroup *cgrp,
         /* Advance to the next non-empty css_set */
         do {
                 l = l->next;
-                if (l == &cgrp->cset_links) {
+                if (l == &it->origin_cgrp->cset_links) {
                         it->cset_link = NULL;
                         return;
                 }
@@ -3257,21 +3255,22 @@ void cgroup_task_iter_start(struct cgroup *cgrp, struct cgroup_task_iter *it)
                 cgroup_enable_task_cg_lists();
 
         read_lock(&css_set_lock);
+
+        it->origin_cgrp = cgrp;
         it->cset_link = &cgrp->cset_links;
-        cgroup_advance_task_iter(cgrp, it);
+
+        cgroup_advance_task_iter(it);
 }
 
 /**
  * cgroup_task_iter_next - return the next task for the iterator
- * @cgrp: the cgroup to walk tasks of
  * @it: the task iterator being iterated
  *
  * The "next" function for task iteration.  @it should have been
  * initialized via cgroup_task_iter_start().  Returns NULL when the
  * iteration reaches the end.
  */
-struct task_struct *cgroup_task_iter_next(struct cgroup *cgrp,
-                                          struct cgroup_task_iter *it)
+struct task_struct *cgroup_task_iter_next(struct cgroup_task_iter *it)
 {
         struct task_struct *res;
         struct list_head *l = it->task;
@@ -3289,7 +3288,7 @@ struct task_struct *cgroup_task_iter_next(struct cgroup *cgrp,
                  * We reached the end of this task list - move on to the
                  * next cgrp_cset_link.
                  */
-                cgroup_advance_task_iter(cgrp, it);
+                cgroup_advance_task_iter(it);
         } else {
                 it->task = l;
         }
@@ -3298,12 +3297,11 @@ struct task_struct *cgroup_task_iter_next(struct cgroup *cgrp,
 
 /**
  * cgroup_task_iter_end - finish task iteration
- * @cgrp: the cgroup to walk tasks of
  * @it: the task iterator to finish
  *
  * Finish task iteration started by cgroup_task_iter_start().
  */
-void cgroup_task_iter_end(struct cgroup *cgrp, struct cgroup_task_iter *it)
+void cgroup_task_iter_end(struct cgroup_task_iter *it)
         __releases(css_set_lock)
 {
         read_unlock(&css_set_lock);
@@ -3409,7 +3407,7 @@ int cgroup_scan_tasks(struct cgroup_scanner *scan)
          */
         heap->size = 0;
         cgroup_task_iter_start(scan->cgrp, &it);
-        while ((p = cgroup_task_iter_next(scan->cgrp, &it))) {
+        while ((p = cgroup_task_iter_next(&it))) {
                 /*
                  * Only affect tasks that qualify per the caller's callback,
                  * if he provided one
@@ -3442,7 +3440,7 @@ int cgroup_scan_tasks(struct cgroup_scanner *scan)
                  * the heap and wasn't inserted
                  */
         }
-        cgroup_task_iter_end(scan->cgrp, &it);
+        cgroup_task_iter_end(&it);
 
         if (heap->size) {
                 for (i = 0; i < heap->size; i++) {
@@ -3664,7 +3662,7 @@ static int pidlist_array_load(struct cgroup *cgrp, enum cgroup_filetype type,
                 return -ENOMEM;
         /* now, populate the array */
         cgroup_task_iter_start(cgrp, &it);
-        while ((tsk = cgroup_task_iter_next(cgrp, &it))) {
+        while ((tsk = cgroup_task_iter_next(&it))) {
                 if (unlikely(n == length))
                         break;
                 /* get tgid or pid for procs or tasks file respectively */
@@ -3675,7 +3673,7 @@ static int pidlist_array_load(struct cgroup *cgrp, enum cgroup_filetype type,
                 if (pid > 0) /* make sure to only use valid results */
                         array[n++] = pid;
         }
-        cgroup_task_iter_end(cgrp, &it);
+        cgroup_task_iter_end(&it);
         length = n;
         /* now sort & (if procs) strip out duplicates */
         sort(array, length, sizeof(pid_t), cmppid, NULL);
@@ -3724,7 +3722,7 @@ int cgroupstats_build(struct cgroupstats *stats, struct dentry *dentry)
         cgrp = dentry->d_fsdata;
 
         cgroup_task_iter_start(cgrp, &it);
-        while ((tsk = cgroup_task_iter_next(cgrp, &it))) {
+        while ((tsk = cgroup_task_iter_next(&it))) {
                 switch (tsk->state) {
                 case TASK_RUNNING:
                         stats->nr_running++;
@@ -3744,7 +3742,7 @@ int cgroupstats_build(struct cgroupstats *stats, struct dentry *dentry)
                         break;
                 }
         }
-        cgroup_task_iter_end(cgrp, &it);
+        cgroup_task_iter_end(&it);
 
 err:
         return ret;
diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index c9177f8fc661..e0ab9bfd679a 100644
--- a/kernel/cgroup_freezer.c
+++ b/kernel/cgroup_freezer.c
@@ -281,7 +281,7 @@ static void update_if_frozen(struct cgroup_subsys_state *css)
         /* are all tasks frozen? */
         cgroup_task_iter_start(css->cgroup, &it);
 
-        while ((task = cgroup_task_iter_next(css->cgroup, &it))) {
+        while ((task = cgroup_task_iter_next(&it))) {
                 if (freezing(task)) {
                         /*
                          * freezer_should_skip() indicates that the task
@@ -296,7 +296,7 @@ static void update_if_frozen(struct cgroup_subsys_state *css)
 
         freezer->state |= CGROUP_FROZEN;
 out_iter_end:
-        cgroup_task_iter_end(css->cgroup, &it);
+        cgroup_task_iter_end(&it);
 out_unlock:
         spin_unlock_irq(&freezer->lock);
 }
@@ -327,9 +327,9 @@ static void freeze_cgroup(struct freezer *freezer)
         struct task_struct *task;
 
         cgroup_task_iter_start(cgroup, &it);
-        while ((task = cgroup_task_iter_next(cgroup, &it)))
+        while ((task = cgroup_task_iter_next(&it)))
                 freeze_task(task);
-        cgroup_task_iter_end(cgroup, &it);
+        cgroup_task_iter_end(&it);
 }
 
 static void unfreeze_cgroup(struct freezer *freezer)
@@ -339,9 +339,9 @@ static void unfreeze_cgroup(struct freezer *freezer)
         struct task_struct *task;
 
         cgroup_task_iter_start(cgroup, &it);
-        while ((task = cgroup_task_iter_next(cgroup, &it)))
+        while ((task = cgroup_task_iter_next(&it)))
                 __thaw_task(task);
-        cgroup_task_iter_end(cgroup, &it);
+        cgroup_task_iter_end(&it);
 }
 
 /**
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 00b055dd1d3d..5a5f4dc649f0 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1804,7 +1804,7 @@ static void mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
                 struct task_struct *task;
 
                 cgroup_task_iter_start(cgroup, &it);
-                while ((task = cgroup_task_iter_next(cgroup, &it))) {
+                while ((task = cgroup_task_iter_next(&it))) {
                         switch (oom_scan_process_thread(task, totalpages, NULL,
                                                         false)) {
                         case OOM_SCAN_SELECT:
@@ -1817,7 +1817,7 @@ static void mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
                         case OOM_SCAN_CONTINUE:
                                 continue;
                         case OOM_SCAN_ABORT:
-                                cgroup_task_iter_end(cgroup, &it);
+                                cgroup_task_iter_end(&it);
                                 mem_cgroup_iter_break(memcg, iter);
                                 if (chosen)
                                         put_task_struct(chosen);
@@ -1834,7 +1834,7 @@ static void mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
                                 get_task_struct(chosen);
                         }
                 }
-                cgroup_task_iter_end(cgroup, &it);
+                cgroup_task_iter_end(&it);
         }
 
         if (!chosen)