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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>
This commit is contained in:
Tejun Heo 2012-11-19 08:13:37 -08:00
parent d19e19de48
commit b8a2df6a5b
1 changed files with 9 additions and 3 deletions
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12
kernel/cgroup.c
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@ -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;