[PATCH 5/6] cgroup: update locking in cgroup_show_options()

Tejun Heo
2014-01-17T18:12:44Z

cgroup_show_options() grabs cgroup_root_mutex to protect the options
changing while printing; however, holding root_mutex or not doesn't
really make much difference for the function. subsys_mask can be
atomically tested and most of the options aren't allowed to change
anyway once mounted.

The only field which needs synchronization is ->release_agent_path.
This patch introduces a dedicated spinlock to synchronize accesses to
the field and drops cgroup_root_mutex locking from
cgroup_show_options(). The next patch will remove cgroup_root_mutex.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
kernel/cgroup.c | 19 ++++++++++++++++---
1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 3b2fda0..2736689 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -92,6 +92,12 @@ static DEFINE_MUTEX(cgroup_mutex);

static DEFINE_MUTEX(cgroup_root_mutex);

+/*
+ * Protects cgroup_subsys->release_agent_path. Modifying it also requires
+ * cgroup_mutex. Reading requires either cgroup_mutex or this spinlock.
+ */
+static DEFINE_SPINLOCK(release_agent_path_lock);
+
#define cgroup_assert_mutex_or_rcu_locked() \
rcu_lockdep_assert(rcu_read_lock_held() || \
lockdep_is_held(&cgroup_mutex), \
@@ -1024,7 +1030,6 @@ static int cgroup_show_options(struct seq_file *seq, struct dentry *dentry)
struct cgroup_subsys *ss;
int ssid;

- mutex_lock(&cgroup_root_mutex);
for_each_subsys(ss, ssid)
if (root->subsys_mask & (1 << ssid))
seq_printf(seq, ",%s", ss->name);
@@ -1034,13 +1039,16 @@ static int cgroup_show_options(struct seq_file *seq, struct dentry *dentry)
seq_puts(seq, ",noprefix");
if (root->flags & CGRP_ROOT_XATTR)
seq_puts(seq, ",xattr");
+
+ spin_lock(&release_agent_path_lock);
if (strlen(root->release_agent_path))
seq_printf(seq, ",release_agent=%s", root->release_agent_path);
+ spin_unlock(&release_agent_path_lock);
+
if (test_bit(CGRP_CPUSET_CLONE_CHILDREN, &root->top_cgroup.flags))
seq_puts(seq, ",clone_children");
if (strlen(root->name))
seq_printf(seq, ",name=%s", root->name);
- mutex_unlock(&cgroup_root_mutex);
return 0;
}

@@ -1262,8 +1270,11 @@ static int cgroup_remount(struct super_block *sb, int *flags, char *data)
if (ret)
goto out_unlock;

- if (opts.release_agent)
+ if (opts.release_agent) {
+ spin_lock(&release_agent_path_lock);
strcpy(root->release_agent_path, opts.release_agent);
+ spin_unlock(&release_agent_path_lock);
+ }
out_unlock:
kfree(opts.release_agent);
kfree(opts.name);
@@ -2172,7 +2183,9 @@ static int cgroup_release_agent_write(struct cgroup_subsys_state *css,
if (!cgroup_lock_live_group(css->cgroup))
return -ENODEV;
mutex_lock(&cgroup_root_mutex);
+ spin_lock(&release_agent_path_lock);
strcpy(css->cgroup->root->release_agent_path, buffer);
+ spin_unlock(&release_agent_path_lock);
mutex_unlock(&cgroup_root_mutex);
mutex_unlock(&cgroup_mutex);
return 0;
--
1.8.4.2