gpu: Add GpuFence framework.

ui/gfx/shared_event.cc

+// Copyright 2016 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 "base/atomicops.h"
+#include "base/macros.h"
+#include "base/process/memory.h"
+#include "base/time/time.h"
+#include "ui/gfx/gfx_export.h"
+#include "ui/gfx/shared_event.h"
+
+#if defined(OS_LINUX) || defined(OS_ANDROID)
+#include <linux/futex.h>
+#include <stdint.h>
+#include <sys/syscall.h>
+#else
+#include "base/threading/platform_thread.h"
+#endif
+
+namespace gfx {
+namespace {
+
+enum { NOT_SIGNALED = 0, WAITING, SIGNALED };
+
+constexpr size_t kSharedMemorySize = sizeof(int32_t);

[ericrk, 2017/03/10 03:04:39]

I'm curious about the size efficiency here. From my (limited) understanding of
the shared memory system on some of the Chrome platforms, the minimum size
returnable is a page (~4k), and allocating smaller will just allocate the page
and give you a subset. This would make creating a bunch of 4 byte shared memory
allocations pretty inefficient. Is that something we should consider here? Is
there a piece of this I'm misunderstanding?

[reveman, 2017/03/13 12:33:05]

Correct. Each fence will end up using one page of memory. That's not ideal but
I'm not sure we'll be creating enough fences on platforms without a native fence
implementation that this will be a problem. We can try to keep multiple fences
in the same chunk of shared memory but I would prefer to keep this simple as a
start and just use one page per fence.

+
+}  // namespace
+
+SharedEvent::SharedEvent() {
+  bool rv = shared_memory_.CreateAndMapAnonymous(kSharedMemorySize);
+  CHECK(rv);
+}
+
+SharedEvent::SharedEvent(const SharedEventHandle& handle)
+    : shared_memory_(handle, false) {
+  bool rv = shared_memory_.Map(kSharedMemorySize);
+  CHECK(rv);
+}
+
+// static
+SharedEventHandle SharedEvent::CreateForProcess(base::ProcessHandle process) {
+  base::SharedMemory shared_memory;
+  if (!shared_memory.CreateAnonymous(kSharedMemorySize))
+    return SharedEventHandle();
+  base::SharedMemoryHandle handle;
+  shared_memory.GiveToProcess(process, &handle);
+  return handle;
+}
+
+// static
+SharedEventHandle SharedEvent::DuplicateHandle(
+    const SharedEventHandle& handle) {
+  return base::SharedMemory::DuplicateHandle(handle);
+}
+
+void SharedEvent::Reset() {
+  // Change state to un-signaled iff event is currently in signaled state.
+  base::subtle::NoBarrier_CompareAndSwap(uaddr(), SIGNALED, NOT_SIGNALED);
+}
+
+void SharedEvent::Signal() {
+  // Change state to signaled and return early unless waiters exist that need
+  // to be woken up.
+  if (base::subtle::Release_CompareAndSwap(uaddr(), NOT_SIGNALED, SIGNALED) !=
+      WAITING)
+    return;
+
+  // Unconditionally change state to signaled before waking up waiters.
+  base::subtle::Release_Store(uaddr(), SIGNALED);
+
+#if defined(OS_LINUX) || defined(OS_ANDROID)
+  // Perform non-blocking futex operation that will wake up all waiters.
+  int rv = syscall(SYS_futex, uaddr(), FUTEX_WAKE, INT_MAX, NULL, NULL, 0);
+  PLOG_IF(ERROR, rv < 0) << "futex";
+#endif
+}
+
+bool SharedEvent::IsSignaled() {
+  return base::subtle::Acquire_Load(uaddr()) == SIGNALED;
+}
+
+bool SharedEvent::Wait(const base::TimeDelta& max_time) {
+#if defined(OS_LINUX) || defined(OS_ANDROID)
+  struct timespec timeout = max_time.ToTimeSpec();
+#endif
+
+  // Wait for signaled state by repeatadly attempting to enter waiting state
+  // until it fails because current state is signaled.
+  while (base::subtle::Acquire_CompareAndSwap(uaddr(), NOT_SIGNALED, WAITING) !=
+         SIGNALED) {
+#if defined(OS_LINUX) || defined(OS_ANDROID)
+    // Perform futex operation that will block until woken up or |timeout| has
+    // expired. Note: If the futex value does not match waiting state, then the
+    // call fails immediately with the error EAGAIN.
+    if (syscall(SYS_futex, uaddr(), FUTEX_WAIT, WAITING, &timeout, NULL, 0)) {
+      PLOG_IF(ERROR, errno != EAGAIN && errno != ETIMEDOUT) << "futex";
+      return false;
+    }
+    // Protect against spurious wake-ups by checking state again. Reset
+    // timeout to prevent this function from blocking significantly longer
+    // than |max_time|.
+    timeout = {0};
+#else
+    base::PlatformThread::YieldCurrentThread();
+    return false;
+#endif
+  }
+
+  return true;
+}
+
+SharedEventHandle SharedEvent::GetHandle() const {
+  return shared_memory_.handle();
+}
+
+void SharedEvent::Close() {
+  shared_memory_.Close();
+}
+
+}  // namespace gfx