cgroup: refactor cgroup_pidlist_find()

Rename cgroup_pidlist_find() to cgroup_pidlist_find_create() and
separate out finding proper to cgroup_pidlist_find().  Also, move
locking to the caller.

This patch is preparation for pidlist restructure and doesn't
introduce any behavior changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

1 files changed, 38 insertions, 27 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index ef019c075d5d..d58c30d3b828 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3575,48 +3575,50 @@ static int cmppid(const void *a, const void *b)
         return *(pid_t *)a - *(pid_t *)b;
 }
 
+static struct cgroup_pidlist *cgroup_pidlist_find(struct cgroup *cgrp,
+                                                  enum cgroup_filetype type)
+{
+        struct cgroup_pidlist *l;
+        /* don't need task_nsproxy() if we're looking at ourself */
+        struct pid_namespace *ns = task_active_pid_ns(current);
+
+        lockdep_assert_held(&cgrp->pidlist_mutex);
+
+        list_for_each_entry(l, &cgrp->pidlists, links)
+                if (l->key.type == type && l->key.ns == ns)
+                        return l;
+        return NULL;
+}
+
 /*
  * find the appropriate pidlist for our purpose (given procs vs tasks)
  * returns with the lock on that pidlist already held, and takes care
  * of the use count, or returns NULL with no locks held if we're out of
  * memory.
  */
-static struct cgroup_pidlist *cgroup_pidlist_find(struct cgroup *cgrp,
-                                                  enum cgroup_filetype type)
+static struct cgroup_pidlist *cgroup_pidlist_find_create(struct cgroup *cgrp,
+                                                enum cgroup_filetype type)
 {
         struct cgroup_pidlist *l;
-        /* don't need task_nsproxy() if we're looking at ourself */
-        struct pid_namespace *ns = task_active_pid_ns(current);
 
-        /*
-         * We can't drop the pidlist_mutex before taking the l->rwsem in case
-         * the last ref-holder is trying to remove l from the list at the same
-         * time. Holding the pidlist_mutex precludes somebody taking whichever
-         * list we find out from under us - compare release_pid_array().
-         */
-        mutex_lock(&cgrp->pidlist_mutex);
-        list_for_each_entry(l, &cgrp->pidlists, links) {
-                if (l->key.type == type && l->key.ns == ns) {
-                        /* make sure l doesn't vanish out from under us */
-                        down_write(&l->rwsem);
-                        mutex_unlock(&cgrp->pidlist_mutex);
-                        return l;
-                }
-        }
+        lockdep_assert_held(&cgrp->pidlist_mutex);
+
+        l = cgroup_pidlist_find(cgrp, type);
+        if (l)
+                return l;
+
         /* entry not found; create a new one */
         l = kzalloc(sizeof(struct cgroup_pidlist), GFP_KERNEL);
-        if (!l) {
-                mutex_unlock(&cgrp->pidlist_mutex);
+        if (!l)
                 return l;
-        }
+
         init_rwsem(&l->rwsem);
         INIT_DELAYED_WORK(&l->destroy_dwork, cgroup_pidlist_destroy_work_fn);
-        down_write(&l->rwsem);
         l->key.type = type;
-        l->key.ns = get_pid_ns(ns);
+        /* don't need task_nsproxy() if we're looking at ourself */
+        l->key.ns = get_pid_ns(task_active_pid_ns(current));
         l->owner = cgrp;
         list_add(&l->links, &cgrp->pidlists);
-        mutex_unlock(&cgrp->pidlist_mutex);
         return l;
 }
 
@@ -3662,17 +3664,26 @@ static int pidlist_array_load(struct cgroup *cgrp, enum cgroup_filetype type,
         sort(array, length, sizeof(pid_t), cmppid, NULL);
         if (type == CGROUP_FILE_PROCS)
                 length = pidlist_uniq(array, length);
-        l = cgroup_pidlist_find(cgrp, type);
+
+        mutex_lock(&cgrp->pidlist_mutex);
+
+        l = cgroup_pidlist_find_create(cgrp, type);
         if (!l) {
+                mutex_unlock(&cgrp->pidlist_mutex);
                 pidlist_free(array);
                 return -ENOMEM;
         }
-        /* store array, freeing old if necessary - lock already held */
+
+        /* store array, freeing old if necessary */
+        down_write(&l->rwsem);
         pidlist_free(l->list);
         l->list = array;
         l->length = length;
         l->use_count++;
         up_write(&l->rwsem);
+
+        mutex_unlock(&cgrp->pidlist_mutex);
+
         *lp = l;
         return 0;
 }