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authorTejun Heo <tj@kernel.org>2012-11-19 08:13:37 -0800
committerTejun Heo <tj@kernel.org>2012-11-19 08:13:37 -0800
commitb8a2df6a5b576d04f78f54abf254c283527d8bbc (patch)
tree6a49e4105d3bb3d813378d72ebf67c08d719d0de
parentd19e19de48aa0b90c56cd93c8a46ebac46273429 (diff)
downloadlinux-b8a2df6a5b576d04f78f54abf254c283527d8bbc.tar.bz2
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>
-rw-r--r--kernel/cgroup.c12
1 files changed, 9 insertions, 3 deletions
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 166b5141f3d4..c53f42e31704 100644
--- 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;