[dpdk-dev] [PATCH v8 3/4] doc/rcu: add lib_rcu documentation

[Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>]

Add lib_rcu QSBR API and programmer guide documentation.

Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Marko Kovacevic <marko.kovacevic@intel.com>
---
 doc/api/doxy-api-index.md                     |   3 +-
 doc/api/doxy-api.conf.in                      |   1 +
 .../prog_guide/img/rcu_general_info.svg       | 509 ++++++++++++++++++
 doc/guides/prog_guide/index.rst               |   1 +
 doc/guides/prog_guide/rcu_lib.rst             | 185 +++++++
 5 files changed, 698 insertions(+), 1 deletion(-)
 create mode 100644 doc/guides/prog_guide/img/rcu_general_info.svg
 create mode 100644 doc/guides/prog_guide/rcu_lib.rst

diff --git a/doc/api/doxy-api-index.md b/doc/api/doxy-api-index.md
index de1e215dd..8f0e84de6 100644
--- a/doc/api/doxy-api-index.md
+++ b/doc/api/doxy-api-index.md
@@ -54,7 +54,8 @@ The public API headers are grouped by topics:
   [memzone]            (@ref rte_memzone.h),
   [mempool]            (@ref rte_mempool.h),
   [malloc]             (@ref rte_malloc.h),
-  [memcpy]             (@ref rte_memcpy.h)
+  [memcpy]             (@ref rte_memcpy.h),
+  [rcu]                (@ref rte_rcu_qsbr.h)
 
 - **timers**:
   [cycles]             (@ref rte_cycles.h),
diff --git a/doc/api/doxy-api.conf.in b/doc/api/doxy-api.conf.in
index 7722fc3e9..b9896cb63 100644
--- a/doc/api/doxy-api.conf.in
+++ b/doc/api/doxy-api.conf.in
@@ -51,6 +51,7 @@ INPUT                   = @TOPDIR@/doc/api/doxy-api-index.md \
                           @TOPDIR@/lib/librte_port \
                           @TOPDIR@/lib/librte_power \
                           @TOPDIR@/lib/librte_rawdev \
+                          @TOPDIR@/lib/librte_rcu \
                           @TOPDIR@/lib/librte_reorder \
                           @TOPDIR@/lib/librte_ring \
                           @TOPDIR@/lib/librte_sched \
diff --git a/doc/guides/prog_guide/img/rcu_general_info.svg b/doc/guides/prog_guide/img/rcu_general_info.svg
new file mode 100644
index 000000000..e7ca1dacb
--- /dev/null
+++ b/doc/guides/prog_guide/img/rcu_general_info.svg
@@ -0,0 +1,509 @@
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+		<g id="shape29-71" v:mID="29" v:groupContext="shape" transform="translate(419.545,-660.119)">
+			<title>Sheet.29</title>
+			<path d="M0 1145.17 L0 1188 L0 1145.17" class="st7"/>
+		</g>
+		<g id="shape30-74" v:mID="30" v:groupContext="shape" transform="translate(419.545,-684.783)">
+			<title>Sheet.30</title>
+			<path d="M0 1188 L82.7 1187.36 L151.2 1172.07" class="st7"/>
+		</g>
+		<g id="shape31-77" v:mID="31" v:groupContext="shape" transform="translate(214.454,-663.095)">
+			<title>Sheet.31</title>
+			<desc>Remove reference to entry1</desc>
+			<v:textBlock v:margins="rect(0,0,0,0)"/>
+			<v:textRect cx="96.7728" cy="1169.45" width="193.55" height="37.1049"/>
+			<path d="M193.55 1150.9 L0 1150.9 L0 1188 L193.55 1188 L193.55 1150.9" class="st3"/>
+			<text x="2.39" y="1163.15" class="st8" v:langID="1033"><v:paragraph v:horizAlign="2"/><v:tabList/>Remove reference <tspan
+						x="104.08" dy="1.2em" class="st9">to entry1</tspan></text>		</g>
+		<g id="shape33-82" v:mID="33" v:groupContext="shape" transform="translate(571.287,-681.326)">
+			<title>Sheet.33</title>
+			<path d="M0 738.67 L0 1188" class="st10"/>
+		</g>
+		<g id="shape34-85" v:mID="34" v:groupContext="shape" transform="translate(515.013,-1130.65)">
+			<title>Sheet.34</title>
+			<desc>Delete</desc>
+			<v:textBlock v:margins="rect(0,0,0,0)"/>
+			<v:textRect cx="60.7243" cy="1166.58" width="121.45" height="42.8314"/>
+			<path d="M121.45 1145.17 L0 1145.17 L0 1188 L121.45 1188 L121.45 1145.17" class="st3"/>
+			<text x="26.02" y="1174.2" class="st11" v:langID="1033"><v:paragraph v:horizAlign="1"/><v:tabList/>Delete</text>		</g>
+		<g id="shape35-89" v:mID="35" v:groupContext="shape" transform="translate(434.372,-1096.8)">
+			<title>Sheet.35</title>
+			<path d="M0 1154.35 L0 1188 L0 1154.35" class="st7"/>
+		</g>
+		<g id="shape36-92" v:mID="36" v:groupContext="shape" transform="translate(434.372,-1100.37)">
+			<title>Sheet.36</title>
+			<path d="M0 1171.88 L84.54 1171.24 L136.43 1188" class="st7"/>
+		</g>
+		<g id="shape37-95" v:mID="37" v:groupContext="shape" transform="translate(193.5,-1103.76)">
+			<title>Sheet.37</title>
+			<desc>Delete entry1 from D1</desc>
+			<v:textBlock v:margins="rect(0,0,0,0)"/>
+			<v:textRect cx="114.75" cy="1175.76" width="229.5" height="24.4771"/>
+			<path d="M229.5 1163.52 L0 1163.52 L0 1188 L229.5 1188 L229.5 1163.52" class="st3"/>
+			<text x="3.88" y="1182.06" class="st8" v:langID="1033"><v:paragraph v:horizAlign="1"/><v:tabList/>Delete entry1 from D1</text>		</g>
+		<g id="shape38-99" v:mID="38" v:groupContext="shape" transform="translate(714.3,-675.425)">
+			<title>Sheet.38</title>
+			<path d="M0 732.77 L0 1188" class="st10"/>
+		</g>
+		<g id="shape39-102" v:mID="39" v:groupContext="shape" transform="translate(795.979,-637.904)">
+			<title>Sheet.39</title>
+			<path d="M0 1112.54 L0 1188 L0 1112.54" class="st7"/>
+		</g>
+		<g id="shape40-105" v:mID="40" v:groupContext="shape" transform="translate(716.782,-675.425)">
+			<title>Sheet.40</title>
+			<path d="M79.2 1188 L52.71 1187.94 L0 1147.21" class="st7"/>
+		</g>
+		<g id="shape41-108" v:mID="41" v:groupContext="shape" transform="translate(803.572,-639.285)">
+			<title>Sheet.41</title>
+			<desc>Free memory for entries1 and 2 after every reader has gone th...</desc>
+			<v:textBlock v:margins="rect(0,0,0,0)"/>
+			<v:textRect cx="172.421" cy="1152.51" width="344.85" height="70.9752"/>
+			<path d="M344.84 1117.02 L0 1117.02 L0 1188 L344.84 1188 L344.84 1117.02" class="st3"/>
+			<text x="0" y="1133.61" class="st8" v:langID="1033"><v:paragraph/><v:tabList/>Free memory for entries1 and 2 <tspan
+						x="0" dy="1.2em" class="st9">after every reader has gone </tspan><tspan x="0" dy="1.2em" class="st9">through at least 1 quiescent state </tspan> </text>		</g>
+		<g id="shape46-114" v:mID="46" v:groupContext="shape" transform="translate(680.801,-1130.65)">
+			<title>Sheet.46</title>
+			<desc>Free</desc>
+			<v:textBlock v:margins="rect(0,0,0,0)"/>
+			<v:textRect cx="42.0169" cy="1166.58" width="84.04" height="42.8314"/>
+			<path d="M84.03 1145.17 L0 1145.17 L0 1188 L84.03 1188 L84.03 1145.17" class="st3"/>
+			<text x="18.89" y="1174.2" class="st11" v:langID="1033"><v:paragraph v:horizAlign="1"/><v:tabList/>Free</text>		</g>
+		<g id="shape48-118" v:mID="48" v:groupContext="shape" transform="translate(811.005,-1110.05)">
+			<title>Sheet.48</title>
+			<path d="M0 1145.17 L0 1188 L0 1145.17" class="st7"/>
+		</g>
+		<g id="shape49-121" v:mID="49" v:groupContext="shape" transform="translate(658.61,-1083.99)">
+			<title>Sheet.49</title>
+			<path d="M153.05 1149.63 L113.7 1149.57 L0 1188" class="st7"/>
+		</g>
+		<g id="shape50-124" v:mID="50" v:groupContext="shape" transform="translate(798.359,-1110.46)">
+			<title>Sheet.50</title>
+			<desc>Grace Period</desc>
+			<v:textBlock v:margins="rect(0,0,0,0)"/>
+			<v:textRect cx="107.799" cy="1167.81" width="215.6" height="40.3845"/>
+			<path d="M215.6 1147.62 L0 1147.62 L0 1188 L215.6 1188 L215.6 1147.62" class="st3"/>
+			<text x="43.79" y="1174.99" class="st6" v:langID="1033"><v:paragraph v:horizAlign="1"/><v:tabList/>Grace Period</text>		</g>
+		<g id="shape51-128" v:mID="51" v:groupContext="shape" transform="translate(599.196,-662.779)">
+			<title>Sheet.51</title>
+			<path d="M0 732.77 L0 1188" class="st12"/>
+		</g>
+		<g id="shape52-131" v:mID="52" v:groupContext="shape" transform="translate(464.931,-1052.95)">
+			<title>Sheet.52</title>
+			<path d="M0 1154.35 L0 1188 L0 1154.35" class="st7"/>
+		</g>
+		<g id="shape53-134" v:mID="53" v:groupContext="shape" transform="translate(464.931,-1056.52)">
+			<title>Sheet.53</title>
+			<path d="M0 1171.88 L84.54 1171.24 L136.43 1188" class="st7"/>
+		</g>
+		<g id="shape54-137" v:mID="54" v:groupContext="shape" transform="translate(225,-1058.76)">
+			<title>Sheet.54</title>
+			<desc>Delete entry2 from D1</desc>
+			<v:textBlock v:margins="rect(0,0,0,0)"/>
+			<v:textRect cx="114.75" cy="1175.76" width="229.5" height="24.4771"/>
+			<path d="M229.5 1163.52 L0 1163.52 L0 1188 L229.5 1188 L229.5 1163.52" class="st3"/>
+			<text x="3.88" y="1182.06" class="st8" v:langID="1033"><v:paragraph v:horizAlign="1"/><v:tabList/>Delete entry2 from D1</text>		</g>
+		<g id="shape56-141" v:mID="56" v:groupContext="shape" transform="translate(711.244,-662.779)">
+			<title>Sheet.56</title>
+			<path d="M0 732.77 L0 1188" class="st12"/>
+		</g>
+		<g id="shape57-144" v:mID="57" v:groupContext="shape" transform="translate(664.897,-1045.31)">
+			<title>Sheet.57</title>
+			<path d="M-0 1188 L146.76 1112.94" class="st13"/>
+		</g>
+		<g id="shape58-147" v:mID="58" v:groupContext="shape" transform="translate(619.059,-848.701)">
+			<title>Sheet.58</title>
+			<path d="M432.2 1184.24 L-0 1188" class="st7"/>
+		</g>
+		<g id="shape59-150" v:mID="59" v:groupContext="shape" transform="translate(1038.62,-837.364)">
+			<title>Sheet.59</title>
+			<desc>Critical sections</desc>
+			<v:textBlock v:margins="rect(0,0,0,0)"/>
+			<v:textRect cx="130" cy="1167.81" width="260.01" height="40.3845"/>
+			<path d="M260 1147.62 L0 1147.62 L0 1188 L260 1188 L260 1147.62" class="st3"/>
+			<text x="52.25" y="1174.99" class="st6" v:langID="1033"><v:paragraph v:horizAlign="1"/><v:tabList/>Critical sections</text>		</g>
+		<g id="shape60-154" v:mID="60" v:groupContext="shape" transform="translate(621.606,-848.828)">
+			<title>Sheet.60</title>
+			<path d="M0 1173.53 L0 1188" class="st7"/>
+		</g>
+		<g id="shape61-157" v:mID="61" v:groupContext="shape" transform="translate(824.31,-849.848)">
+			<title>Sheet.61</title>
+			<path d="M0 1173.53 L0 1188" class="st7"/>
+		</g>
+		<g id="shape62-160" v:mID="62" v:groupContext="shape" transform="translate(345.944,-933.143)">
+			<title>Sheet.62</title>
+			<path d="M705.32 1188 L0 1187.43" class="st7"/>
+		</g>
+		<g id="shape63-163" v:mID="63" v:groupContext="shape" transform="translate(1038.62,-915.684)">
+			<title>Sheet.63</title>
+			<desc>Quiescent states</desc>
+			<v:textBlock v:margins="rect(0,0,0,0)"/>
+			<v:textRect cx="137.691" cy="1167.81" width="275.39" height="40.3845"/>
+			<path d="M275.38 1147.62 L0 1147.62 L0 1188 L275.38 1188 L275.38 1147.62" class="st3"/>
+			<text x="55.18" y="1174.99" class="st6" v:langID="1033"><v:paragraph v:horizAlign="1"/><v:tabList/>Quiescent states</text>		</g>
+		<g id="shape64-167" v:mID="64" v:groupContext="shape" transform="translate(346.581,-932.442)">
+			<title>Sheet.64</title>
+			<path d="M0 1173.53 L0 1188" class="st7"/>
+		</g>
+		<g id="shape65-170" v:mID="65" v:groupContext="shape" transform="translate(621.606,-933.461)">
+			<title>Sheet.65</title>
+			<path d="M0 1173.53 L0 1188" class="st7"/>
+		</g>
+		<g id="shape66-173" v:mID="66" v:groupContext="shape" transform="translate(856.905,-934.481)">
+			<title>Sheet.66</title>
+			<path d="M0 1173.53 L0 1188" class="st7"/>
+		</g>
+		<g id="shape67-176" v:mID="67" v:groupContext="shape" transform="translate(472.82,-756.389)">
+			<title>Sheet.67</title>
+			<path d="M578.44 1188 L0 1187.43" class="st7"/>
+		</g>
+		<g id="shape68-179" v:mID="68" v:groupContext="shape" transform="translate(473.456,-755.688)">
+			<title>Sheet.68</title>
+			<path d="M0 1173.53 L0 1188" class="st7"/>
+		</g>
+		<g id="shape69-182" v:mID="69" v:groupContext="shape" transform="translate(1016.87,-757.728)">
+			<title>Sheet.69</title>
+			<path d="M0 1173.53 L0 1188" class="st7"/>
+		</g>
+		<g id="shape70-185" v:mID="70" v:groupContext="shape" transform="translate(1060.04,-738.651)">
+			<title>Sheet.70</title>
+			<desc>while(1) loop</desc>
+			<v:textBlock v:margins="rect(0,0,0,0)"/>
+			<v:textRect cx="96.7728" cy="1167.81" width="193.55" height="40.3845"/>
+			<path d="M193.55 1147.62 L0 1147.62 L0 1188 L193.55 1188 L193.55 1147.62" class="st3"/>
+			<text x="31.03" y="1174.99" class="st6" v:langID="1033"><v:paragraph v:horizAlign="1"/><v:tabList/>while(1) loop</text>		</g>
+		<g id="shape71-189" v:mID="71" v:groupContext="shape" transform="translate(190.02,-464.886)">
+			<title>Sheet.71</title>
+			<path d="M0 1151.91 C0 1148.19 3.88 1145.17 8.66 1145.17 L43.29 1145.17 C48.13 1145.17 51.95 1148.19 51.95 1151.91 L51.95
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+					class="st1"/>
+		</g>
+		<g id="shape72-191" v:mID="72" v:groupContext="shape" transform="translate(259.003,-466.895)">
+			<title>Sheet.72</title>
+			<desc>Reader thread is not accessing any shared data structure. i.e...</desc>
+			<v:textBlock v:margins="rect(0,0,0,0)"/>
+			<v:textRect cx="344.967" cy="1169.45" width="689.94" height="37.1049"/>
+			<path d="M689.93 1150.9 L0 1150.9 L0 1188 L689.93 1188 L689.93 1150.9" class="st3"/>
+			<text x="0" y="1162.25" class="st14" v:langID="1033"><v:paragraph/><v:tabList/>Reader thread is not accessing any shared data structure.<v:newlineChar/><tspan
+						x="0" dy="1.2em" class="st9">i.e. non critical section or quiescent state.</tspan></text>		</g>
+		<g id="shape73-196" v:mID="73" v:groupContext="shape" transform="translate(190.02,-389.169)">
+			<title>Sheet.73</title>
+			<desc>Dx</desc>
+			<v:textBlock v:margins="rect(4,4,4,4)"/>
+			<v:textRect cx="25.9746" cy="1166.58" width="51.95" height="42.8314"/>
+			<path d="M0 1152.31 C0 1148.39 1.43 1145.17 3.16 1145.17 L48.79 1145.17 C50.55 1145.17 51.95 1148.39 51.95 1152.31 L51.95
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+					class="st2"/>
+			<text x="12.9" y="1173.78" class="st15" v:langID="1033"><v:paragraph v:horizAlign="1"/><v:tabList/>Dx</text>		</g>
+		<g id="shape74-199" v:mID="74" v:groupContext="shape" transform="translate(259.003,-388.777)">
+			<title>Sheet.74</title>
+			<desc>Reader thread is accessing the shared data structure Dx. i.e....</desc>
+			<v:textBlock v:margins="rect(0,0,0,0)"/>
+			<v:textRect cx="344.967" cy="1169.45" width="689.94" height="37.1049"/>
+			<path d="M689.93 1150.9 L0 1150.9 L0 1188 L689.93 1188 L689.93 1150.9" class="st3"/>
+			<text x="0" y="1162.25" class="st14" v:langID="1033"><v:paragraph/><v:tabList/>Reader thread is accessing the shared data structure Dx.<v:newlineChar/><tspan
+						x="0" dy="1.2em" class="st9">i.e. critical section.</tspan></text>		</g>
+		<g id="shape75-204" v:mID="75" v:groupContext="shape" transform="translate(289.017,-301.151)">
+			<title>Sheet.75</title>
+			<desc>Point in time when the reference to the entry is removed usin...</desc>
+			<v:textBlock v:margins="rect(0,0,0,0)"/>
+			<v:textRect cx="332.491" cy="1160.47" width="664.99" height="55.0626"/>
+			<path d="M664.98 1132.94 L0 1132.94 L0 1188 L664.98 1188 L664.98 1132.94" class="st3"/>
+			<text x="0" y="1153.27" class="st14" v:langID="1033"><v:paragraph/><v:tabList/>Point in time when the reference to the entry is removed <tspan
+						x="0" dy="1.2em" class="st9">using an atomic operation.</tspan></text>		</g>
+		<g id="shape76-209" v:mID="76" v:groupContext="shape" transform="translate(177.543,-315.596)">
+			<title>Sheet.76</title>
+			<desc>Delete</desc>
+			<v:textBlock v:margins="rect(0,0,0,0)"/>
+			<v:textRect cx="45.9546" cy="1166.58" width="91.91" height="42.8314"/>
+			<path d="M91.91 1145.17 L0 1145.17 L0 1188 L91.91 1188 L91.91 1145.17" class="st3"/>
+			<text x="11.25" y="1174.2" class="st11" v:langID="1033"><v:paragraph v:horizAlign="1"/><v:tabList/>Delete</text>		</g>
+		<g id="shape77-213" v:mID="77" v:groupContext="shape" transform="translate(288,-239.327)">
+			<title>Sheet.77</title>
+			<desc>Point in time when the writer can free the deleted entry.</desc>
+			<v:textBlock v:margins="rect(0,0,0,0)"/>
+			<v:textRect cx="328.5" cy="1167.81" width="657" height="40.3845"/>
+			<path d="M657 1147.62 L0 1147.62 L0 1188 L657 1188 L657 1147.62" class="st3"/>
+			<text x="0" y="1175.01" class="st14" v:langID="1033"><v:paragraph/><v:tabList/>Point in time when the writer can free the deleted entry.</text>		</g>
+		<g id="shape78-217" v:mID="78" v:groupContext="shape" transform="translate(177.543,-240.744)">
+			<title>Sheet.78</title>
+			<desc>Free</desc>
+			<v:textBlock v:margins="rect(0,0,0,0)"/>
+			<v:textRect cx="34.3786" cy="1166.58" width="68.76" height="42.8314"/>
+			<path d="M68.76 1145.17 L0 1145.17 L0 1188 L68.76 1188 L68.76 1145.17" class="st3"/>
+			<text x="11.25" y="1174.2" class="st11" v:langID="1033"><v:paragraph v:horizAlign="1"/><v:tabList/>Free</text>		</g>
+		<g id="shape79-221" v:mID="79" v:groupContext="shape" transform="translate(289.228,-163.612)">
+			<title>Sheet.79</title>
+			<desc>Time duration between Delete and Free, during which memory ca...</desc>
+			<v:textBlock v:margins="rect(0,0,0,0)"/>
+			<v:textRect cx="328.5" cy="1167.81" width="657" height="40.3845"/>
+			<path d="M657 1147.62 L0 1147.62 L0 1188 L657 1188 L657 1147.62" class="st3"/>
+			<text x="0" y="1160.61" class="st14" v:langID="1033"><v:paragraph/><v:tabList/>Time duration between Delete and Free, during which <tspan
+						x="0" dy="1.2em" class="st9">memory cannot be freed.</tspan></text>		</g>
+		<g id="shape80-226" v:mID="80" v:groupContext="shape" transform="translate(187.999,-162)">
+			<title>Sheet.80</title>
+			<desc>Grace Period</desc>
+			<v:textBlock v:margins="rect(0,0,0,0)"/>
+			<v:textRect cx="39.5985" cy="1166.58" width="79.2" height="42.8314"/>
+			<path d="M79.2 1145.17 L0 1145.17 L0 1188 L79.2 1188 L79.2 1145.17" class="st3"/>
+			<text x="0" y="1158.96" class="st11" v:langID="1033"><v:paragraph/><v:tabList/>Grace <tspan x="0" dy="1.2em"
+						class="st9">Period</tspan></text>		</g>
+		<g id="shape83-231" v:mID="83" v:groupContext="shape" transform="translate(572.146,-1080.07)">
+			<title>Sheet.83</title>
+			<path d="M9.3 1188 L9.66 1188 L132.49 1188" class="st16"/>
+		</g>
+		<g id="shape84-240" v:mID="84" v:groupContext="shape" transform="translate(599.196,-1042.14)">
+			<title>Sheet.84</title>
+			<path d="M7.41 1188 L7.77 1188 L104.28 1188" class="st18"/>
+		</g>
+		<g id="shape85-249" v:mID="85" v:groupContext="shape" transform="translate(980.637,-595.338)">
+			<title>Sheet.85</title>
+			<path d="M0 1188 L92.16 1188" class="st20"/>
+		</g>
+		<g id="shape86-254" v:mID="86" v:groupContext="shape" transform="translate(444.835,-603.428)">
+			<title>Sheet.86</title>
+			<path d="M0 1145.17 L0 1188 L0 1145.17" class="st7"/>
+		</g>
+		<g id="shape87-257" v:mID="87" v:groupContext="shape" transform="translate(444.835,-637.489)">
+			<title>Sheet.87</title>
+			<path d="M0 1188 L84.43 1186.61 L154.36 1153.31" class="st7"/>
+		</g>
+		<g id="shape88-260" v:mID="88" v:groupContext="shape" transform="translate(241.369,-607.028)">
+			<title>Sheet.88</title>
+			<desc>Remove reference to entry2</desc>
+			<v:textBlock v:margins="rect(0,0,0,0)"/>
+			<v:textRect cx="96.7728" cy="1169.45" width="193.55" height="37.1049"/>
+			<path d="M193.55 1150.9 L0 1150.9 L0 1188 L193.55 1188 L193.55 1150.9" class="st3"/>
+			<text x="2.39" y="1163.15" class="st8" v:langID="1033"><v:paragraph v:horizAlign="2"/><v:tabList/>Remove reference <tspan
+						x="104.08" dy="1.2em" class="st9">to entry2</tspan></text>		</g>
+	</g>
+</svg>
diff --git a/doc/guides/prog_guide/index.rst b/doc/guides/prog_guide/index.rst
index 95f5e7964..17df2c563 100644
--- a/doc/guides/prog_guide/index.rst
+++ b/doc/guides/prog_guide/index.rst
@@ -56,6 +56,7 @@ Programmer's Guide
     metrics_lib
     bpf_lib
     ipsec_lib
+    rcu_lib
     source_org
     dev_kit_build_system
     dev_kit_root_make_help
diff --git a/doc/guides/prog_guide/rcu_lib.rst b/doc/guides/prog_guide/rcu_lib.rst
new file mode 100644
index 000000000..55d44e15d
--- /dev/null
+++ b/doc/guides/prog_guide/rcu_lib.rst
@@ -0,0 +1,185 @@
+..  SPDX-License-Identifier: BSD-3-Clause
+    Copyright(c) 2019 Arm Limited.
+
+.. _RCU_Library:
+
+RCU Library
+============
+
+Lock-less data structures provide scalability and determinism.
+They enable use cases where locking may not be allowed
+(for ex: real-time applications).
+
+In the following paras, the term 'memory' refers to memory allocated
+by typical APIs like malloc or anything that is representative of
+memory, for ex: an index of a free element array.
+
+Since these data structures are lock less, the writers and readers
+are accessing the data structures concurrently. Hence, while removing
+an element from a data structure, the writers cannot return the memory
+to the allocator, without knowing that the readers are not
+referencing that element/memory anymore. Hence, it is required to
+separate the operation of removing an element into 2 steps:
+
+Delete: in this step, the writer removes the reference to the element from
+the data structure but does not return the associated memory to the
+allocator. This will ensure that new readers will not get a reference to
+the removed element. Removing the reference is an atomic operation.
+
+Free(Reclaim): in this step, the writer returns the memory to the
+memory allocator, only after knowing that all the readers have stopped
+referencing the deleted element.
+
+This library helps the writer determine when it is safe to free the
+memory.
+
+This library makes use of thread Quiescent State (QS).
+
+What is Quiescent State
+-----------------------
+Quiescent State can be defined as 'any point in the thread execution where the
+thread does not hold a reference to shared memory'. It is up to the application
+to determine its quiescent state.
+
+Let us consider the following diagram:
+
+.. figure:: img/rcu_general_info.*
+
+
+As shown, reader thread 1 accesses data structures D1 and D2. When it is
+accessing D1, if the writer has to remove an element from D1, the
+writer cannot free the memory associated with that element immediately.
+The writer can return the memory to the allocator only after the reader
+stops referencing D1. In other words, reader thread RT1 has to enter
+a quiescent state.
+
+Similarly, since reader thread 2 is also accessing D1, writer has to
+wait till thread 2 enters quiescent state as well.
+
+However, the writer does not need to wait for reader thread 3 to enter
+quiescent state. Reader thread 3 was not accessing D1 when the delete
+operation happened. So, reader thread 1 will not have a reference to the
+deleted entry.
+
+It can be noted that, the critical sections for D2 is a quiescent state
+for D1. i.e. for a given data structure Dx, any point in the thread execution
+that does not reference Dx is a quiescent state.
+
+Since memory is not freed immediately, there might be a need for
+provisioning of additional memory, depending on the application requirements.
+
+Factors affecting RCU mechanism
+---------------------------------
+
+It is important to make sure that this library keeps the overhead of
+identifying the end of grace period and subsequent freeing of memory,
+to a minimum. The following paras explain how grace period and critical
+section affect this overhead.
+
+The writer has to poll the readers to identify the end of grace period.
+Polling introduces memory accesses and wastes CPU cycles. The memory
+is not available for reuse during grace period. Longer grace periods
+exasperate these conditions.
+
+The length of the critical section and the number of reader threads
+is proportional to the duration of the grace period. Keeping the critical
+sections smaller will keep the grace period smaller. However, keeping the
+critical sections smaller requires additional CPU cycles (due to additional
+reporting) in the readers.
+
+Hence, we need the characteristics of small grace period and large critical
+section. This library addresses this by allowing the writer to do
+other work without having to block till the readers report their quiescent
+state.
+
+RCU in DPDK
+-----------
+
+For DPDK applications, the start and end of while(1) loop (where no
+references to shared data structures are kept) act as perfect quiescent
+states. This will combine all the shared data structure accesses into a
+single, large critical section which helps keep the overhead on the
+reader side to a minimum.
+
+DPDK supports pipeline model of packet processing and service cores.
+In these use cases, a given data structure may not be used by all the
+workers in the application. The writer does not have to wait for all
+the workers to report their quiescent state. To provide the required
+flexibility, this library has a concept of QS variable. The application
+can create one QS variable per data structure to help it track the
+end of grace period for each data structure. This helps keep the grace
+period to a minimum.
+
+How to use this library
+-----------------------
+
+The application must allocate memory and initialize a QS variable.
+
+Application can call ``rte_rcu_qsbr_get_memsize`` to calculate the size
+of memory to allocate. This API takes maximum number of reader threads,
+using this variable, as a parameter. Currently, a maximum of 1024 threads
+are supported.
+
+Further, the application can initialize a QS variable using the API
+``rte_rcu_qsbr_init``.
+
+Each reader thread is assumed to have a unique thread ID. Currently, the
+management of the thread ID (for ex: allocation/free) is left to the
+application. The thread ID should be in the range of 0 to
+maximum number of threads provided while creating the QS variable.
+The application could also use lcore_id as the thread ID where applicable.
+
+``rte_rcu_qsbr_thread_register`` API will register a reader thread
+to report its quiescent state. This can be called from a reader thread.
+A control plane thread can also call this on behalf of a reader thread.
+The reader thread must call ``rte_rcu_qsbr_thread_online`` API to start
+reporting its quiescent state.
+
+Some of the use cases might require the reader threads to make
+blocking API calls (for ex: while using eventdev APIs). The writer thread
+should not wait for such reader threads to enter quiescent state.
+The reader thread must call ``rte_rcu_qsbr_thread_offline`` API, before calling
+blocking APIs. It can call ``rte_rcu_qsbr_thread_online`` API once the blocking
+API call returns.
+
+The writer thread can trigger the reader threads to report their quiescent
+state by calling the API ``rte_rcu_qsbr_start``. It is possible for multiple
+writer threads to query the quiescent state status simultaneously. Hence,
+``rte_rcu_qsbr_start`` returns a token to each caller.
+
+The writer thread must call ``rte_rcu_qsbr_check`` API with the token to
+get the current quiescent state status. Option to block till all the reader
+threads enter the quiescent state is provided. If this API indicates that
+all the reader threads have entered the quiescent state, the application
+can free the deleted entry.
+
+The APIs ``rte_rcu_qsbr_start`` and ``rte_rcu_qsbr_check`` are lock free.
+Hence, they can be called concurrently from multiple writers even while
+running as worker threads.
+
+The separation of triggering the reporting from querying the status provides
+the writer threads flexibility to do useful work instead of blocking for the
+reader threads to enter the quiescent state or go offline. This reduces the
+memory accesses due to continuous polling for the status.
+
+``rte_rcu_qsbr_synchronize`` API combines the functionality of
+``rte_rcu_qsbr_start`` and blocking ``rte_rcu_qsbr_check`` into a single API.
+This API triggers the reader threads to report their quiescent state and
+polls till all the readers enter the quiescent state or go offline. This
+API does not allow the writer to do useful work while waiting and
+introduces additional memory accesses due to continuous polling.
+
+The reader thread must call ``rte_rcu_qsbr_thread_offline`` and
+``rte_rcu_qsbr_thread_unregister`` APIs to remove itself from reporting its
+quiescent state. The ``rte_rcu_qsbr_check`` API will not wait for this reader
+thread to report the quiescent state status anymore.
+
+The reader threads should call ``rte_rcu_qsbr_quiescent`` API to indicate that
+they entered a quiescent state. This API checks if a writer has triggered a
+quiescent state query and update the state accordingly.
+
+The ``rte_rcu_qsbr_lock`` and ``rte_rcu_qsbr_unlock`` are empty functions.
+However, when ``CONFIG_RTE_LIBRTE_RCU_DEBUG`` is enabled, these APIs aid
+in debugging issues. One can mark the access to shared data structures on the
+reader side using these APIs. The ``rte_rcu_qsbr_quiescent`` will check if
+all the locks are unlocked.
-- 
2.17.1