cgroup: use mutex_trylock() when grabbing i_mutex of a new cgroup directory

All cgroup directory i_mutexes nest outside cgroup_mutex; however, new
directory creation is a special case.  A new cgroup directory is
created while holding cgroup_mutex.  Populating the new directory
requires both the new directory's i_mutex and cgroup_mutex.  Because
all directory i_mutexes nest outside cgroup_mutex, grabbing both
requires releasing cgroup_mutex first, which isn't a good idea as the
new cgroup isn't yet ready to be manipulated by other cgroup
opreations.

This is worked around by grabbing the new directory's i_mutex while
holding cgroup_mutex before making it visible.  As there's no other
user at that point, grabbing the i_mutex under cgroup_mutex can't lead
to deadlock.

cgroup_create_file() was using I_MUTEX_CHILD to tell lockdep not to
worry about the reverse locking order; however, this creates pseudo
locking dependency cgroup_mutex -> I_MUTEX_CHILD, which isn't true -
all directory i_mutexes are still nested outside cgroup_mutex.  This
pseudo locking dependency can lead to spurious lockdep warnings.

Use mutex_trylock() instead.  This will always succeed and lockdep
doesn't create any locking dependency for it.

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 166b5141f3d441b1c33942a7c999390285af633e..c53f42e317043b5f57215301b771ae324f954c2a 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2657,9 +2657,15 @@ static int cgroup_create_file(struct dentry *dentry, umode_t mode,
                inc_nlink(inode);
                inc_nlink(dentry->d_parent->d_inode);
 
-               /* start with the directory inode held, so that we can
-                * populate it without racing with another mkdir */
-               mutex_lock_nested(&inode->i_mutex, I_MUTEX_CHILD);
+               /*
+                * Control reaches here with cgroup_mutex held.
+                * @inode->i_mutex should nest outside cgroup_mutex but we
+                * want to populate it immediately without releasing
+                * cgroup_mutex.  As @inode isn't visible to anyone else
+                * yet, trylock will always succeed without affecting
+                * lockdep checks.
+                */
+               WARN_ON_ONCE(!mutex_trylock(&inode->i_mutex));
        } else if (S_ISREG(mode)) {
                inode->i_size = 0;
                inode->i_fop = &cgroup_file_operations;