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cgroup: saner refcounting for cgroup_root
* make the reference from superblock to cgroup_root counting - do cgroup_put() in cgroup_kill_sb() whether we'd done percpu_ref_kill() or not; matching grab is done when we allocate a new root. That gives the same refcounting rules for all callers of cgroup_do_mount() - a reference to cgroup_root has been grabbed by caller and it either is transferred to new superblock or dropped. * have cgroup_kill_sb() treat an already killed refcount as "just don't bother killing it, then". * after successful cgroup_do_mount() have cgroup1_mount() recheck if we'd raced with mount/umount from somebody else and cgroup_root got killed. In that case we drop the superblock and bugger off with -ERESTARTSYS, same as if we'd found it in the list already dying. * don't bother with delayed initialization of refcount - it's unreliable and not needed. No need to prevent attempts to bump the refcount if we find cgroup_root of another mount in progress - sget will reuse an existing superblock just fine and if the other sb manages to die before we get there, we'll catch that immediately after cgroup_do_mount(). * don't bother with kernfs_pin_sb() - no need for doing that either. Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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Al Viro
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Jan 17, 2019
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