cgroup: remove cgroup_pidlist->rwsem

cgroup_pidlist locking is needlessly complicated.  It has outer
cgroup->pidlist_mutex to protect the list of pidlists associated with
a cgroup and then each pidlist has rwsem to synchronize updates and
reads.  Given that the only read access is from seq_file operations
which are always invoked back-to-back, the rwsem is a giant overkill.
All it does is adding unnecessary complexity.

This patch removes cgroup_pidlist->rwsem and protects all accesses to
pidlists belonging to a cgroup with cgroup->pidlist_mutex.
pidlist->rwsem locking is removed if it's nested inside
cgroup->pidlist_mutex; otherwise, it's replaced with
cgroup->pidlist_mutex locking.

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

1 files changed, 4 insertions, 11 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index d58c30d3b828..dc39e1774542 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3462,8 +3462,6 @@ struct cgroup_pidlist {
         struct list_head links;
         /* pointer to the cgroup we belong to, for list removal purposes */
         struct cgroup *owner;
-        /* protects the other fields */
-        struct rw_semaphore rwsem;
         /* for delayed destruction */
         struct delayed_work destroy_dwork;
 };
@@ -3522,7 +3520,6 @@ static void cgroup_pidlist_destroy_work_fn(struct work_struct *work)
         struct cgroup_pidlist *tofree = NULL;
 
         mutex_lock(&l->owner->pidlist_mutex);
-        down_write(&l->rwsem);
 
         /*
          * Destroy iff we didn't race with a new user or get queued again.
@@ -3535,7 +3532,6 @@ static void cgroup_pidlist_destroy_work_fn(struct work_struct *work)
                 tofree = l;
         }
 
-        up_write(&l->rwsem);
         mutex_unlock(&l->owner->pidlist_mutex);
         kfree(tofree);
 }
@@ -3612,7 +3608,6 @@ static struct cgroup_pidlist *cgroup_pidlist_find_create(struct cgroup *cgrp,
         if (!l)
                 return l;
 
-        init_rwsem(&l->rwsem);
         INIT_DELAYED_WORK(&l->destroy_dwork, cgroup_pidlist_destroy_work_fn);
         l->key.type = type;
         /* don't need task_nsproxy() if we're looking at ourself */
@@ -3675,12 +3670,10 @@ static int pidlist_array_load(struct cgroup *cgrp, enum cgroup_filetype type,
         }
 
         /* 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);
 
@@ -3762,7 +3755,7 @@ static void *cgroup_pidlist_start(struct seq_file *s, loff_t *pos)
         int index = 0, pid = *pos;
         int *iter;
 
-        down_read(&l->rwsem);
+        mutex_lock(&of->cgrp->pidlist_mutex);
         if (pid) {
                 int end = l->length;
 
@@ -3790,7 +3783,7 @@ static void cgroup_pidlist_stop(struct seq_file *s, void *v)
 {
         struct cgroup_pidlist_open_file *of = s->private;
 
-        up_read(&of->pidlist->rwsem);
+        mutex_unlock(&of->cgrp->pidlist_mutex);
 }
 
 static void *cgroup_pidlist_next(struct seq_file *s, void *v, loff_t *pos)
@@ -3830,13 +3823,13 @@ static const struct seq_operations cgroup_pidlist_seq_operations = {
 
 static void cgroup_release_pid_array(struct cgroup_pidlist *l)
 {
-        down_write(&l->rwsem);
+        mutex_lock(&l->owner->pidlist_mutex);
         BUG_ON(!l->use_count);
         /* if the last user, arm the destroy work */
         if (!--l->use_count)
                 mod_delayed_work(cgroup_pidlist_destroy_wq, &l->destroy_dwork,
                                  CGROUP_PIDLIST_DESTROY_DELAY);
-        up_write(&l->rwsem);
+        mutex_unlock(&l->owner->pidlist_mutex);
 }
 
 static int cgroup_pidlist_release(struct inode *inode, struct file *file)