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Commit 069df3b7 authored by Tejun Heo's avatar Tejun Heo
Browse files

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: default avatarTejun Heo <tj@kernel.org>
Acked-by: default avatarLi Zefan <lizefan@huawei.com>
parent e6b81710

kernel/cgroup.c

+4 −11
Original line number Diff line number Diff line
@@ -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)