From: Paul Menage <menage@xxxxxxxxxx>
I think this is the right way to handle the lockdep false-positive in the
current cgroups patches, but I'm not that familiar with lockdep so any
suggestions for a better approach are welcomed.
In order to avoid a false-positive lockdep warning, we lock the root inode
of a new filesystem mount prior to taking cgroup_mutex, to preserve the
invariant that cgroup_mutex nests inside inode->i_mutex. In order to
prevent a lockdep false positive when locking i_mutex on a newly-created
cgroup directory inode we use mutex_lock_nested(), with a nesting level
of I_MUTEX_CHILD since the new inode will ultimately be a child directory
of the parent whose i_mutex is nested outside of cgroup_mutex.
Signed-off-by: Paul Menage <menage@xxxxxxxxxx>
Acked-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
kernel/cgroup.c | 17 +++++++----------
1 files changed, 7 insertions(+), 10 deletions(-)
diff -puN kernel/cgroup.c~task-cgroupsv11-shared-cgroup-subsystem-group-arrays-avoid-lockdep-warning kernel/cgroup.c
--- a/kernel/cgroup.c~task-cgroupsv11-shared-cgroup-subsystem-group-arrays-avoid-lockdep-warning
+++ a/kernel/cgroup.c
@@ -867,13 +867,16 @@ static int cgroup_get_sb(struct file_
} else {
/* New superblock */
struct cgroup *cont = &root->top_cgroup;
+ struct inode *inode;
BUG_ON(sb->s_root != NULL);
ret = cgroup_get_rootdir(sb);
if (ret)
goto drop_new_super;
+ inode = sb->s_root->d_inode;
+ mutex_lock(&inode->i_mutex);
mutex_lock(&cgroup_mutex);
/*
@@ -886,12 +889,14 @@ static int cgroup_get_sb(struct file_
ret = allocate_cg_links(css_set_count, &tmp_cg_links);
if (ret) {
mutex_unlock(&cgroup_mutex);
+ mutex_unlock(&inode->i_mutex);
goto drop_new_super;
}
ret = rebind_subsystems(root, root->subsys_bits);
if (ret == -EBUSY) {
mutex_unlock(&cgroup_mutex);
+ mutex_unlock(&inode->i_mutex);
goto drop_new_super;
}
@@ -931,16 +936,8 @@ static int cgroup_get_sb(struct file_
BUG_ON(!list_empty(&cont->children));
BUG_ON(root->number_of_cgroups != 1);
- /*
- * I believe that it's safe to nest i_mutex inside
- * cgroup_mutex in this case, since no-one else can
- * be accessing this directory yet. But we still need
- * to teach lockdep that this is the case - currently
- * a cgroupfs remount triggers a lockdep warning
- */
- mutex_lock(&cont->dentry->d_inode->i_mutex);
cgroup_populate_dir(cont);
- mutex_unlock(&cont->dentry->d_inode->i_mutex);
+ mutex_unlock(&inode->i_mutex);
mutex_unlock(&cgroup_mutex);
}
@@ -1358,7 +1355,7 @@ static int cgroup_create_file(struct
/* start with the directory inode held, so that we can
* populate it without racing with another mkdir */
- mutex_lock(&inode->i_mutex);
+ mutex_lock_nested(&inode->i_mutex, I_MUTEX_CHILD);
} else if (S_ISREG(mode)) {
inode->i_size = 0;
inode->i_fop = &cgroup_file_operations;
_
--
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