cgroup: use mutex_trylock() when grabbing i_mutex of a new cgroup directory

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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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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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kernel/cgroup.c

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