Import userspace-rcu-0.15.0 from https://liburcu.org/ needed by bind-…

…9.20.4.

This is a userspace implentation of the read/copy/update synchronization
mechanism described in https://docs.kernel.org/RCU/whatisRCU.html.

external/lgpl2/userspace-rcu/dist/.gitreview

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+# SPDX-FileCopyrightText: 2020 EfficiOS Inc.
+#
+# SPDX-License-Identifier: MIT
+
+[gerrit]
+host=review.lttng.org
+port=29418
+project=userspace-rcu.git
+defaultbranch=master
+defaultremote=review
+defaultrebase=0

external/lgpl2/userspace-rcu/dist/CodingStyle.md

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+<!--
+SPDX-FileCopyrightText: 2012 Mathieu Desnoyers <[email protected]>
+
+SPDX-License-Identifier: CC-BY-4.0
+-->
+
+# Userspace RCU Coding Style
+
+The coding style used for this project follows the Linux kernel
+guide lines. Please refer to:
+
+- Linux kernel Documentation/CodingStyle document for details,
+- Linux kernel scripts/checkpatch.pl for a script which verify the patch
+  coding style.
+
+Mathieu Desnoyers, May 30, 2012

external/lgpl2/userspace-rcu/dist/LICENSE.md

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+# Userspace RCU library licensing
+
+This project is compliant with the [REUSE](https://reuse.software/spec/)
+specification, each file contains [SPDX](https://spdx.org/specifications) tags,
+the full licenses can be found in the `LICENSES/` directory.
+
+
+## Library code
+
+The library part is distributed under `LGPL-2.1-or-later`. See
+`LICENSES/LGPL-2.1-or-later.txt` for license details. Refer to the individual
+file headers for details.
+
+LGPL-compatible source code can statically use the library header using :
+
+````
+#define _LGPL_SOURCE
+#include <urcu.h>
+````
+
+Dynamic-only linking with the LGPL library is used if `_LGPL_SOURCE` is not
+defined. It permits relinking with newer versions of the library, which is
+required by the LGPL license.
+
+See `lgpl-relicensing.md` for details.
+
+
+## Headers
+
+Most headers are also distributed under `LGPL-2.1-or-later`, some include code
+from `atomic_ops` and are distributed under `Boehm-GC` and some others are
+distributed under `MIT`. Refer to the individual file headers for details.
+
+xchg() primitive has been rewritten from scratch starting from `atomic_ops 1.2`
+which has a MIT-style license that is intended to allow use in both free and
+proprietary software:
+
+````
+http://www.hpl.hp.com/research/linux/atomic_ops/LICENSING.txt
+http://www.hpl.hp.com/personal/Hans_Boehm/gc/gc_source/
+````
+
+
+## Tests
+
+Library test code is distributed under the `GPL-2.0-only` license. See
+`LICENSES/GPL-2.0-only.txt` for license details. See headers of individual
+files under `tests/` for details.
+
+
+## Documentation
+
+The documentation is distributed under the `CC-BY-4.0` license.
+
+
+## Build system
+
+Build system related files are mostly distributed under the `MIT` license. See
+headers of individual files for details.
+
+
+## Generated data files
+
+Generated data files are distributed under the `CC-1.0` license.

external/lgpl2/userspace-rcu/dist/LICENSES/Autoconf-exception-2.0.txt

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+As a special exception, the Free Software Foundation gives unlimited permission to copy, distribute and modify the configure scripts that are the output of Autoconf. You need not follow the terms of the GNU General Public License when using or distributing such scripts, even though portions of the text of Autoconf appear in them. The GNU General Public License (GPL) does govern all other use of the material that constitutes the Autoconf program.
+
+Certain portions of the Autoconf source text are designed to be copied (in certain cases, depending on the input) into the output of Autoconf. We call these the "data" portions. The rest of the Autoconf source text consists of comments plus executable code that decides which of the data portions to output in any given case. We call these comments and executable code the "non-data" portions. Autoconf never copies any of the non-data portions into its output.
+
+This special exception to the GPL applies to versions of Autoconf released by the Free Software Foundation. When you make and distribute a modified version of Autoconf, you may extend this special exception to the GPL to apply to your modified version as well, *unless* your modified version has the potential to copy into its output some of the text that was the non-data portion of the version that you started with. (In other words, unless your change moves or copies text from the non-data portions to the data portions.) If your modification has such potential, you must delete any notice of this special exception to the GPL from your modified version.

external/lgpl2/userspace-rcu/dist/LICENSES/BSD-2-Clause.txt

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+Copyright (c) <year> <owner> 
+
+Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
+
+1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
+
+2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.