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author | Tejun Heo <tj@kernel.org> | 2012-11-19 08:13:37 -0800 |
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committer | Tejun Heo <tj@kernel.org> | 2012-11-19 08:13:37 -0800 |
commit | b8a2df6a5b576d04f78f54abf254c283527d8bbc (patch) | |
tree | 6a49e4105d3bb3d813378d72ebf67c08d719d0de /kernel | |
parent | d19e19de48aa0b90c56cd93c8a46ebac46273429 (diff) | |
download | linux-b8a2df6a5b576d04f78f54abf254c283527d8bbc.tar.gz |
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>
Diffstat (limited to 'kernel')
-rw-r--r-- | kernel/cgroup.c | 12 |
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; |