Commit b8a2df6a authored Nov 19, 2012 by Tejun Heo

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>

parent d19e19de

kernel/cgroup.c

+9 −3

Original line number	Diff line number	Diff line
		@@ -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;

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