From 2306e3e15fd880f26ba856bd257a3e1ef266777b Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Daniel=20Kr=C3=BCgler?= Date: Sat, 23 Sep 2023 15:39:30 +0200 Subject: [PATCH] New issue from jim x: "The read exclusive ownership of an atomic read-modify-write operation and whether its read and write are two operations are unclear" --- xml/issue3980.xml | 120 ++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 120 insertions(+) create mode 100644 xml/issue3980.xml diff --git a/xml/issue3980.xml b/xml/issue3980.xml new file mode 100644 index 0000000000..f621ca39ad --- /dev/null +++ b/xml/issue3980.xml @@ -0,0 +1,120 @@ + + + + +The read exclusive ownership of an atomic read-modify-write operation and whether its read and write are two operations are unclear +
+jim x +22 Aug 2023 +99 + + +

+Such two questions are sourced from StackOverflow: +

+
    +

  1. Can the read operations in compare_exchange_strong in different two thread read the same value?

    2. +

  2. For purposes of ordering, is atomic read-modify-write one operation or two?

    3. +
+

+Given this example: +

+
+
+#include <iostream>
+#include <atomic>
+#include <thread>
+
+struct SpinLock{
+  std::atomic<bool> atomic_;
+  void lock(){
+    bool expected = false;
+    while (!atomic_.compare_exchange_strong(expected,true,std::memory_order_release,std::memory_order_relaxed)) {
+    }
+  }
+  void unlock(){
+    atomic_.store(false, std::memory_order_release);
+  }
+};
+
+int main(){
+  SpinLock spin{false};
+  auto t1 = std::thread([&](){
+    spin.lock();
+    spin.unlock();
+  });
+  auto t2 = std::thread([&](){
+    spin.lock();
+    spin.unlock();
+  });
+  t1.join();
+  t2.join();
+}
+
+
+

+In the current draft, the relevant phrasing that can interpret that only one read-modify-write operation reads the initial +value false is p10: +

+

+Atomic read-modify-write operations shall always read the last value (in the modification order) written before the write +associated with the read-modify-write operation. +

+

+However, the wording can have two meanings, each kind of read can result in different explanations for the example +

+
    +

  1. The check of the violation is done before the side effect of the RMW is in the modification order, i.e. the rule is +just checked at the read point.

    2. +

  2. The check of the violation is done after the side effect of the RMW is in the modification order, i.e. the rule is +checked when RMW tries to add the side effect that is based on the read-value to the modification order, and that +side effect wouldn't be added to the modification order if the rule was violated.

    3. +
+

+With the first interpretation, the two RMW operations can read the same initial value because that value is indeed the last value +in the modification order before such two RMW operations produce the side effect to the modification order. +

+With the second interpretation, there is only one RMW operation that can read the initial value because the latter one in +the modification order would violate the rule if it read the initial value. +

+Such two interpretations arise from that the wording doesn't clearly specify when that check is performed. +

+So, my proposed wording is: +

+

+Atomic read-modify-write operations shall always read the value from a side effect X, where X +immediately precedes the side effect of the read-modify-write operation in the modification order. +

+

+This wording keeps a similar utterance to , and it can clearly convey the meaning +that we say the value read by RWM is associated with the side effect of RMW in the modification order. +

+Relevant discussion can be seen CWG/issues/423 here. +

+ + + +

+This wording is relative to . +

+ +
    + +

  1. Modify the as indicated:

    + +
    +

    +-10- Atomic read-modify-write operations shall always read the last value from a side effect X, +where X immediately precedes the side effect of the read-modify-write operation (in the +modification order) written before the write associated with the read-modify-write operation. +

    +-11- Implementations should make atomic stores visible to atomic loads within a reasonable amount of time. +

    +
    + +
    2. + +
+ + +