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>
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index d58c30d..dc39e17 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3462,8 +3462,6 @@
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 @@
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 @@
tofree = l;
}
- up_write(&l->rwsem);
mutex_unlock(&l->owner->pidlist_mutex);
kfree(tofree);
}
@@ -3612,7 +3608,6 @@
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 @@
}
/* 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 @@
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 @@
{
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 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)
