From 824c3aa83d9acc45567421c0d08ca6eb4d718a43 Mon Sep 17 00:00:00 2001
From: Johannes Stoelp <johannes.stoelp@gmail.com>
Date: Wed, 15 Nov 2023 02:36:11 +0100
Subject: post: cas, aba and ll/sc first draft

---
 .../2023-09-01-cas-llsc-aba/cas-ptr-generation.cc  | 134 +++++++++++++++++++++
 1 file changed, 134 insertions(+)
 create mode 100644 content/2023-09-01-cas-llsc-aba/cas-ptr-generation.cc

(limited to 'content/2023-09-01-cas-llsc-aba/cas-ptr-generation.cc')

diff --git a/content/2023-09-01-cas-llsc-aba/cas-ptr-generation.cc b/content/2023-09-01-cas-llsc-aba/cas-ptr-generation.cc
new file mode 100644
index 0000000..319481b
--- /dev/null
+++ b/content/2023-09-01-cas-llsc-aba/cas-ptr-generation.cc
@@ -0,0 +1,134 @@
+#include <cassert>
+#include <cstdint>
+#include <cstdio>
+
+#include <atomic>
+#include <thread>
+
+#include <unistd.h>
+
+#if __x86_64 != 1
+#error "Example only for x86_64"
+#endif
+
+// Only used because we look at the disassembly.
+#define NOINLINE __attribute__((noinline))
+
+// -- UTILITIES ----------------------------------------------------------------
+
+template <typename T>
+NOINLINE bool cmpxchg16b(T* dest, T oldval, T newval) {
+  static_assert(sizeof(T) == 16, "Required for CMPXCHG16B");
+  static_assert(alignof(T) == 16, "Required for CMPXCHG16B");
+
+  const auto as_u128 = [](T* ptr) {
+    return reinterpret_cast<unsigned __int128*>(ptr);
+  };
+  // Use legacy __sync atomic builtin instead __atomic builtin because gcc will
+  // directly emit the 16byte compare exchange instruction instead of emitting a
+  // library call. This is nice when looking at the disassembly, weeh.
+  //
+  // -mcx16
+  //   This option enables GCC to generate "CMPXCHG16B" instructions in 64-bit
+  //   code to implement compare-and- exchange operations on 16-byte aligned
+  //   128-bit objects. This is useful for atomic updates of data
+  //   structures exceeding one machine word in size. The compiler uses this
+  //   instruction to implement __sync Builtins. However, for __atomic Builtins
+  //   operating on 128-bit integers, a library call is always used.
+  return __sync_bool_compare_and_swap(as_u128(dest), *as_u128(&oldval),
+                                      *as_u128(&newval));
+}
+
+// -- BLOCK STACK --------------------------------------------------------------
+
+struct BlockStack {
+  struct Block {
+    Block* next{nullptr};
+    unsigned mem{0};
+  };
+
+  void push(Block* blk) {
+    printf("[%d] push blk->mem=%d\n", gettid(), blk->mem);
+
+    GenerationPtr old_head, new_head;
+    do {
+      old_head     = m_head;
+      blk->next    = old_head.ptr;
+      new_head.ptr = blk;
+      new_head.gen = old_head.gen + 1;
+    } while (!cmpxchg16b(&m_head, old_head, new_head));
+  }
+
+  Block* pop(int s = 0) {
+    GenerationPtr old_head, new_head;
+    do {
+      old_head     = m_head;
+      new_head.ptr = old_head.ptr->next;
+      new_head.gen = old_head.gen + 1;
+
+      // Simulate thread being interrupted.
+      if (s) {
+        sleep(s);
+      }
+    } while (!cmpxchg16b(&m_head, old_head, new_head));
+
+    printf("[%d] pop blk->mem=%d\n", gettid(), old_head.ptr->mem);
+    return old_head.ptr;
+  }
+
+private:
+  struct GenerationPtr {
+    Block* ptr{nullptr};
+    uint64_t gen{0};
+  } __attribute__((aligned(16)));
+
+  GenerationPtr m_head;
+};
+
+// -- MAIN ---------------------------------------------------------------------
+
+int main() {
+  BlockStack::Block B1, B2, B3;
+  B1.mem = 1;
+  B2.mem = 2;
+  B3.mem = 3;
+
+  // Create free memory block list.
+  // S: head -> 1 -> 2 -> 3.
+  BlockStack S;
+  S.push(&B3);
+  S.push(&B2);
+  S.push(&B1);
+
+  auto T1 = std::thread([&S]() {
+    // Read head=1 & next=2, then get interrupted (simulated by sleep).
+    // After resuming, T2 already popped 1 & 2 and pushed again 1 (ABA).
+    //
+    // The first CAS will fail even block 1 is currently at the beginning of the
+    // list as the generation of the OLD_ROOT does not match the generation of
+    // M_ROOT. The CAS loop will retry, read the new M_ROOT, and hence the new
+    // NEXT pointer pointing to 3 and then successfully update M_ROOT.
+    auto* B1 = S.pop(2);
+    assert(B1->mem == 1);
+    // Pops 3, as it should be.
+    auto* B3 = S.pop();
+    assert(B3->mem == 3);
+  });
+
+  auto T2 = std::thread([&S]() {
+    // Artificial sleep, such that T1 entered pop and read head & next.
+    sleep(1);
+
+    // Pop 1 & 2.
+    auto* B1 = S.pop();
+    assert(B1->mem == 1);
+    auto* B2 = S.pop();
+    assert(B2->mem == 2);
+
+    // Re-insert 1.
+    S.push(B1);
+  });
+
+  T2.join();
+  T1.join();
+}
-- 
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