[libc-commits] [libc] [llvm] [libc][CndVar] reimplmement conditional variable with FIFO ordering (PR #192748)

[Alexey Samsonov via libc-commits]

================
@@ -6,49 +6,339 @@
 //
 //===----------------------------------------------------------------------===//
 
-#ifndef LLVM_LIBC___SUPPORT_SRC_THREADS_LINUX_CNDVAR_H
-#define LLVM_LIBC___SUPPORT_SRC_THREADS_LINUX_CNDVAR_H
+#ifndef LLVM_LIBC_SRC___SUPPORT_THREADS_CNDVAR_H
+#define LLVM_LIBC_SRC___SUPPORT_THREADS_CNDVAR_H
 
 #include "hdr/stdint_proxy.h" // uint32_t
+#include "src/__support/CPP/limits.h"
+#include "src/__support/CPP/mutex.h"
+#include "src/__support/CPP/new.h"
 #include "src/__support/macros/config.h"
-#include "src/__support/threads/futex_utils.h"       // Futex
-#include "src/__support/threads/mutex.h"             // Mutex
-#include "src/__support/threads/raw_mutex.h"         // RawMutex
+#include "src/__support/threads/futex_utils.h" // Futex
+#include "src/__support/threads/mutex.h"       // Mutex
+#include "src/__support/threads/raw_mutex.h"   // RawMutex
+#include "src/__support/threads/sleep.h"
+
+#ifdef LIBC_COPT_TIMEOUT_ENSURE_MONOTONICITY
+#include "src/__support/time/monotonicity.h"
+#endif
 
 namespace LIBC_NAMESPACE_DECL {
 
+enum class CndVarResult {
+  Success,
+  MutexError,
+  Timeout,
+};
+
 class CndVar {
-  enum CndWaiterStatus : uint32_t {
-    WS_Waiting = 0xE,
-    WS_Signalled = 0x5,
+public:
+  using Timeout = internal::AbsTimeout;
+
+private:
+  // A single-waiter multiple-notifier barrier used to keep
+  // track of cancellation threads. We use this barrier to
+  // ensure in-queue threads that have posted their cancellation
+  // request have finished dequeue themselves.
+  class CancellationBarrier {
+    LIBC_INLINE_VAR static constexpr size_t CANCEL_STEP = 2;
+    LIBC_INLINE_VAR static constexpr size_t SLEEPING_BIT = 1;
+
+    // LSB indicates whether the waiter is in sleeping state.
+    Futex futex;
+
+  public:
+    LIBC_INLINE CancellationBarrier() : futex(0) {}
+    // Add one more notification request.
+    LIBC_INLINE void add_one() {
+      futex.fetch_add(CANCEL_STEP, cpp::MemoryOrder::RELAXED);
+    }
+    // Send notification to one waiter.
+    LIBC_INLINE void notify() {
+      FutexWordType res = futex.fetch_sub(CANCEL_STEP);
+      // Only need to goto syscall if waiter is sleep and we are the last one
+      if (res <= (CANCEL_STEP | SLEEPING_BIT) && (res & SLEEPING_BIT) != 0)
+        futex.notify_one();
+    }
+    LIBC_INLINE void wait() {
+      size_t spin = 0;
+      while (auto remaining = futex.load(cpp::MemoryOrder::RELAXED)) {
+        // Set LSB to 1 to indicate that the waiter is entering sleeping
+        // state.
+        FutexWordType new_val = remaining | SLEEPING_BIT;
+        if (spin > LIBC_COPT_RAW_MUTEX_DEFAULT_SPIN_COUNT &&
+            futex.compare_exchange_strong(remaining, new_val)) {
+          futex.wait(new_val, /*timeout=*/cpp::nullopt, /*is_pshared=*/false);
+          futex.fetch_sub(1);
+          spin = 0;
+        }
+        sleep_briefly();
+        spin++;
+      }
+    }
+  };
+
+  enum WaiterState : uint8_t {
+    Waiting = 0,
+    Signalled = 1,
+    Cancelled = 2,
+    Requeued = 3,
   };
 
-  struct CndWaiter {
-    Futex futex_word = WS_Waiting;
-    CndWaiter *next = nullptr;
+  struct QueueNode {
+    QueueNode *prev;
+    QueueNode *next;
+
+    // We use cyclic dummy node to avoid handing corner cases.
----------------
vonosmas wrote:

Does this (instead of using `nullptr` actually simplifiy things? E.g. you still have to enforce initialization of the queue, and maintain this tricky invariant (e.g. converting a node into a dummy node when you remove it, and then checking that its .next references itself).

https://github.com/llvm/llvm-project/pull/192748