Revert 146792 - Lock-free GamepadSeqLock (try 2)

content/common/gamepad_seqlock.cc

Index: content/common/gamepad_seqlock.cc
===================================================================
--- content/common/gamepad_seqlock.cc	(revision 146895)
+++ content/common/gamepad_seqlock.cc	(working copy)
@@ -1,87 +1,49 @@
-// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/common/gamepad_seqlock.h"
 
 namespace content {
-namespace internal {
 
-GamepadSeqLockBase::GamepadSeqLockBase(BaseEntry* entries, size_t size)
-    : size_(size)
-    , current_(0) {
-  const size_t ent_size = sizeof(BaseEntry) + sizeof(Atomic32) * size;
-  memset(entries, 0, kEntries * ent_size);
-  for (size_t i = 0; i < kEntries; ++i) {
-    entries_[i] = reinterpret_cast<BaseEntry*>
-        (reinterpret_cast<char*>(entries) + i * ent_size);
-  }
+GamepadSeqLock::GamepadSeqLock()
+  : sequence_(0) {
 }
 
-// The algorithm works as follows.
-// The SeqLock contains 2 user objects - a current and a non-current.
-// The object roles can be swapped by incrementing the current_ variable.
-// Initially both objects are consistent, that is, their sequence_%2 == 0.
-// A writer proceeds as follows. First, it marks the non-current object
-// as inconsistent by incrementing it's sequence_ (the sequence_ becomes odd).
-// Then it mutates the object. Then marks it as consistent by incrementing
-// it's sequence_ once again (the sequence_ is even now). And finally swaps
-// the object roles, that is, the object becomes current.
-// Such disipline establishes an important property - the current object
-// is always consistent and contains the most recent data.
-// Readers proceed as follows. First, determine what is the current object,
-// remember it's seqence, check that the sequence is even
-// (the object is consistent), copy out the object, verify that
-// the sequence is not changed. If any of the checks fail, a reader works
-// with a non-current object (current object is always consistent), so it
-// just retries from the beginning. Thus readers are completely lock-free,
-// that is, a reader retries iff a writer has accomplished a write operation
-// during reading (only constant sequence of writes can stall readers,
-// a stalled writer can't block readers).
-
-void GamepadSeqLockBase::ReadTo(Atomic32* obj) {
-  using base::subtle::NoBarrier_Load;
-  using base::subtle::Acquire_Load;
-  using base::subtle::MemoryBarrier;
+base::subtle::Atomic32 GamepadSeqLock::ReadBegin() {
+  base::subtle::Atomic32 version;
   for (;;) {
-    // Determine the current object.
-    Atomic32 cur = Acquire_Load(&current_);
-    BaseEntry* ent = entries_[cur % kEntries];
-    Atomic32 seq = Acquire_Load(&ent->sequence_);  // membar #LoadLoad
-    // If the counter is even, then the object is not already current,
-    // no need to yield, just retry (the current object is consistent).
-    if (seq % 2)
-      continue;
-    // Copy out the entry.
-    for (size_t i = 0; i < size_; ++i)
-      obj[i] = NoBarrier_Load(&ent->data_[i]);
-    MemoryBarrier();  // membar #LoadLoad
-    Atomic32 seq2 = NoBarrier_Load(&ent->sequence_);
-    // If the counter is changed, then we've read inconsistent data,
-    // no need to yield, just retry (the current object is consistent).
-    if (seq2 != seq)
-      continue;
-    break;
+    version = base::subtle::NoBarrier_Load(&sequence_);
+
+    // If the counter is even, then the associated data might be in a
+    // consistent state, so we can try to read.
+    if ((version & 1) == 0)
+      break;
+
+    // Otherwise, the writer is in the middle of an update. Retry the read.
+    base::PlatformThread::YieldCurrentThread();
   }
+  return version;
 }
 
-void GamepadSeqLockBase::Write(const Atomic32* obj) {
-  using base::subtle::NoBarrier_Store;
-  using base::subtle::Release_Store;
-  using base::subtle::MemoryBarrier;
-  // Get the non current object...
-  BaseEntry* ent = entries_[(current_ + 1) % kEntries];
-  // ... and mark it as inconsistent.
-  NoBarrier_Store(&ent->sequence_, ent->sequence_ + 1);
-  MemoryBarrier();  // membar #StoreStore
-  // Copy in the entry.
-  for (size_t i = 0; i < size_; ++i)
-    NoBarrier_Store(&ent->data_[i], obj[i]);
-  // Mark the object as consistent again.
-  Release_Store(&ent->sequence_, ent->sequence_ + 1);  // membar #StoreStore
-  // Switch the current object.
-  Release_Store(&current_, current_ + 1);
+bool GamepadSeqLock::ReadRetry(base::subtle::Atomic32 version) {
+  // If the sequence number was updated then a read should be re-attempted.
+  // -- Load fence, read membarrier
+  return base::subtle::Release_Load(&sequence_) != version;
 }
 
-}  // namespace internal
-}  // namespace content
+void GamepadSeqLock::WriteBegin() {
+  // Increment the sequence number to odd to indicate the beginning of a write
+  // update.
+  base::subtle::Barrier_AtomicIncrement(&sequence_, 1);
+  // -- Store fence, write membarrier
+}
+
+void GamepadSeqLock::WriteEnd() {
+  // Increment the sequence to an even number to indicate the completion of
+  // a write update.
+  // -- Store fence, write membarrier
+  base::subtle::Barrier_AtomicIncrement(&sequence_, 1);
+}
+
+} // namespace content