[compudj]

IBM allowed their RCU contribution to be used in LGPLv2 or later code through this commit they kindly contributed to the Userspace RCU project:

  commit 54843abcc17c8e8b7600ed635e966c6970d8d20f
  Author: Paul E. McKenney <[e-mail]>
  Date:   Sat May 9 01:11:49 2009 -0400

    LGPL relicensing of IBM's contributions
    
    Add comments noting IBM's permission to relicense its contributions to the
    urcu.h and urcu.c files under the LGPLv2 license, or any later version.
    
    Signed-off-by: Paul E. McKenney <[e-mail]>
    Reviewed-by: Steven L. Bennett <[e-mail]>

[nbsd4life]

That only refers to copyright, not the patent, right? I know FOSS projects avoiding a different implementation of RCU for patent reasons.

[lostmsu]

Doesn't LGPLv2 give explicit patent grant?

[compudj]

GPLv2 provides implicit patent grant in section 6 and section 7. LGPLv2.1 has basically the same content in section 10 and section 11.

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.