That only refers to copyright, not the patent, right? I know FOSS projects avoiding a different implementation of RCU for patent reasons.
IANAL, but AFAIU LGPLv2.1 provides implicit patent grant in the same way as GPLv2.
IBM has allowed use of RCU in GPL code through its contributions to the Linux kernel for many years now. Their contributions to Userspace RCU provide an implicit patent grant in a similar fashion.
Userspace RCU being LGPLv2.1, it allows proprietary applications to link to it, and therefore use RCU through the library APIs, as long as they satisfy LGPLv2.1 requirements.