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