cgroup: use mutex_trylock() when grabbing i_mutex of a new cgroup directory
authorTejun Heo <tj@kernel.org>
Mon, 19 Nov 2012 16:13:37 +0000 (08:13 -0800)
committerTejun Heo <tj@kernel.org>
Mon, 19 Nov 2012 16:13:37 +0000 (08:13 -0800)
commitb8a2df6a5b576d04f78f54abf254c283527d8bbc
tree6a49e4105d3bb3d813378d72ebf67c08d719d0de
parentd19e19de48aa0b90c56cd93c8a46ebac46273429
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>
kernel/cgroup.c