# Userspace RCU
###### tags: `rcu`
- [Userspace RCU - liburcu](https://liburcu.org/)
- [User-space RCU - LWN 2013](https://lwn.net/Articles/573424/)
- [User-space RCU: Atomic-operation and utility API](https://lwn.net/Articles/573435/)
- [User-space RCU: Memory-barrier menagerie](https://lwn.net/Articles/573436/)
- [The user-space RCU API](https://lwn.net/Articles/573439/)
- [The RCU-protected list API](https://lwn.net/Articles/573441/)
- [urcu/userspace-rcu - github](https://github.com/urcu/userspace-rcu)
- [tests/benchmark/](https://github.com/urcu/userspace-rcu/tree/master/tests/benchmark)
- [doc/rcu-api.md](https://github.com/urcu/userspace-rcu/blob/master/doc/rcu-api.md)
- [What Linear Multiprocessor Scalability Means for Your Application](https://blog.linuxplumbersconf.org/2009/slides/Mathieu-Desnoyers-talk-lpc2009.pdf)
Linux Plumbers Conference 2009, slide
- [Userspace RCU library: new APIs and data structures - Mathieu Desnoyers, EfficiOS Inc.](https://youtu.be/yOoWvdBFFRs)
Linux Plumbers Conference 2013, video
- [Mentorship Session: Unraveling RCU-Usage Mysteries (Fundamentals)](https://youtu.be/K-4TI5gFsig)
- [liburcu，一个用户态的RCU实现](https://airekans.github.io/c/2016/05/10/dive-into-liburcu)
- [Userspace RCU原理](https://blog.csdn.net/chenmo187J3X1/article/details/80992945)
---
## URCU overview
When is URCU useful?
TODO
筆記補完整
## RCU Flavors
- QSBR
- RCU_MEMBARRIER
- RCU_MB
- RCU_SIGNAL
- Bullet-proof
### QSBR
- #include <urcu-qsbr.h>
- Link with -lurcu-qsbr
QSBR flavor of RCU
每個 reader thread 都要週期性的呼叫 `rcu_quiescent_state()` 確保程式的 progress。
可以用 rcu_thread_online() and rcu_thread_offline() 使該 reader thread 進入較長時間的 inactive，暫時脫離 grace period 的偵測
由於需要呼叫 `rcu_quiescent_state()`，此 flavor 對於程式的 intrusiveness 較大，但可以得到最快的 read-side speed (zero cost)
----
### RCU_MEMBARRIER
- #include <urcu.h>
- Link the application with -lurcu
> This is the preferred version of the library, in terms of grace-period detection speed, read-side speed and flexibility. Dynamically detects kernel support for sys_membarrier(). Falls back on urcu-mb scheme if support is not present, which has slower read-side.
最推薦的版本，在偵測 grace period, read-side 的速度跟 flexibility 都很好。
會動態偵測是否支援 `sys_membarrier()`，不支援的話會自動退到 RCU_MB 的版本，
，降低 read-side 速度
----
### RCU_MB
- #include <urcu.h>
- Compile any _LGPL_SOURCE code using this library with -DRCU_MB
- Link with -lurcu-mb
在 reader 跟 writer 都會使用 memory barriers，所以在偵測 grace period 的速度較快，但犧牲 reader-side speed
----
### RCU_SIGNAL
- #include <urcu.h>
- Compile any _LGPL_SOURCE code using this library with -DRCU_SIGNAL
- Link the application with -lurcu-signal
> Version of the library that requires a signal, typically SIGUSR1. Can be overridden with -DSIGRCU by modifying Makefile.build.inc.
此 flavor 的 read-side speed 只略輸 QSBR，並且不需要呼叫 quiescent states，但是會需要佔用掉一個 signal，預設是會佔用掉 `SIGUSR1`
----
### Bullet-proof
- #include <urcu-bp.h>
- Link with -lurcu-bp
> It is specifically designed to help tracing library to hook on applications without requiring to modify these applications.
此 flavor 被設計來 hook 在 application 上，並且不用改動這些 applications
前面所有的 flavor 都需要 rcu_init(), rcu_register_thread() and rcu_unregister_thread() 才能夠運作，但這個卻不用。(會轉變成 nops)
可是會犧牲 read-side and write-side 的速度
---
## URCU Test
https://github.com/ccs100203/kernel-module-rcu-test
將此測試移植到 userspace rcu 來做
已經做好了，尚未放到 github 上
TODO, 增加測試的強度 (stress)
### 執行結果:
#### kernel module
```
[348771.405662] id : 0, name : book1, author : jb, borrow : 0, addr : ffff94e2d0490e40
[348771.405667] id : 1, name : book2, author : jb, borrow : 0, addr : ffff94e2d0490240
[348771.405669] book1 borrow : 0
[348771.405669] book2 borrow : 0
[348771.426178] borrow success 0, preempt_count : 0
[348771.453980] borrow success 1, preempt_count : 0
[348771.453985] id : 0, name : book1, author : jb, borrow : 1, addr : ffff94e2d0490900
[348771.453988] id : 1, name : book2, author : jb, borrow : 1, addr : ffff94e49d2adc00
[348771.470186] return success 0, preempt_count : 0
[348771.494184] return success 1, preempt_count : 0
[348771.494189] id : 0, name : book1, author : jb, borrow : 0, addr : ffff94e49d2ad180
[348771.494192] id : 1, name : book2, author : jb, borrow : 0, addr : ffff94e2d0490900
```
#### userspace rcu
目前這裡的 `preempt_count` 目前是直接回傳 0，而不是像 kernel module 使用 [preempt_count() function](https://elixir.bootlin.com/linux/v5.8.9/source/include/asm-generic/preempt.h#L9)
使用到 [urcu/rculist.h](https://github.com/urcu/userspace-rcu/blob/master/include/urcu/rculist.h) 以及 [urcu/list.h](https://github.com/urcu/userspace-rcu/blob/master/include/urcu/list.h)
```
id : 0, name : book1, author : jb, borrow : 0, addr : 55fc4d1cb2a0
id : 1, name : book2, author : jb, borrow : 0, addr : 55fc4d1cb340
book1 borrow : 0
book2 borrow : 0
borrow success 0, preempt_count : 0
borrow success 1, preempt_count : 0
id : 0, name : book1, author : jb, borrow : 1, addr : 55fc4d1cb7f0
id : 1, name : book2, author : jb, borrow : 1, addr : 55fc4d1cb2a0
return success 0, preempt_count : 0
return success 1, preempt_count : 0
id : 0, name : book1, author : jb, borrow : 0, addr : 55fc4d1cb340
id : 1, name : book2, author : jb, borrow : 0, addr : 55fc4d1cb7f0
```
---
## APIs & Docs
- [Userspace RCU API](https://github.com/urcu/userspace-rcu/blob/master/doc/rcu-api.md)
完整 API 連結
- [Userspace RCU Concurrent Data Structures (CDS) API](https://github.com/urcu/userspace-rcu/blob/master/doc/cds-api.md)
This document describes briefly the data structures contained with the userspace RCU library.
### liburcu-defer
### urcu-call-rcu
---
## 已提交 patch
[[lttng-dev] [PATCH] Fix: revise obsolete command in README.md](https://lists.lttng.org/pipermail/lttng-dev/2022-August/030245.html)
測試的時候發現 make bench 早在 2015 被改掉了，被分成 short_bench, long_bench 兩種
但 readme 跟 doc 都沒改
[Refactor tests](https://github.com/urcu/userspace-rcu/commit/ad46005890368f9c306f0c510b3d4b08c47b66f8)
然後 benchmark 沒有紀錄 log 要自己存
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