Add GN arg to enable mutex priority inheritance

Mutex priority inheritance is useful to avoid cases where a
high-priority thread is waiting for a mutex that is held by a low
priority thread that may not get to run for a while. This is
particularly useful for realtime applications. For more discussion, see
https://www.kernel.org/doc/Documentation/pi-futex.txt

Some platforms (eg Chromecast) are known to fulfill the requirements for
safe use of mutex priority inheritance; add a GN arg and buildflag so
that priority inheritance can be enabled for those platforms.

Chromecast uses Linux 4.1 or greater, and glibc 2.23-r10, so the bugs
mentioned in LockImpl::PriorityInheritanceAvailable() are not present.

Bug: 
Change-Id: I21cecd2144d82f038e35aa30a2a51c35d541c644
Reviewed-on: https://chromium-review.googlesource.com/847642
Commit-Queue: Kenneth MacKay <kmackay@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526884}

base/BUILD.gn

@@ -50,6 +50,11 @@ declare_args() {
 
   # Set to true to disable COM init check hooks.
   com_init_check_hook_disabled = false
+
+  # Set to true to enable mutex priority inheritance. See the comments in
+  # LockImpl::PriorityInheritanceAvailable() in lock_impl_posix.cc for the
+  # platform requirements to safely enable priority inheritance.
+  enable_mutex_priority_inheritance = false
 }
 
 if (is_android) {
@@ -1146,6 +1151,7 @@ jumbo_component("base") {
     ":cfi_flags",
     ":debugging_flags",
     ":protected_memory_flags",
+    ":synchronization_flags",
     "//base/numerics:base_numerics",
   ]
 
@@ -1758,6 +1764,14 @@ buildflag_header("protected_memory_flags") {
   flags = [ "USE_LLD=$use_lld" ]
 }
 
+buildflag_header("synchronization_flags") {
+  header = "synchronization_flags.h"
+  header_dir = "base/synchronization"
+
+  flags =
+      [ "ENABLE_MUTEX_PRIORITY_INHERITANCE=$enable_mutex_priority_inheritance" ]
+}
+
 # This is the subset of files from base that should not be used with a dynamic
 # library. Note that this library cannot depend on base because base depends on
 # base_static.

base/synchronization/lock_impl_posix.cc

@@ -8,6 +8,8 @@
 
 #include "base/debug/activity_tracker.h"
 #include "base/synchronization/lock.h"
+#include "base/synchronization/synchronization_flags.h"
+#include "build/build_config.h"
 
 namespace base {
 namespace internal {
@@ -76,11 +78,13 @@ void LockImpl::Lock() {
 
 // static
 bool LockImpl::PriorityInheritanceAvailable() {
-#if PRIORITY_INHERITANCE_LOCKS_POSSIBLE() && defined(OS_MACOSX)
+#if BUILDFLAG(ENABLE_MUTEX_PRIORITY_INHERITANCE)
+  return true;
+#elif PRIORITY_INHERITANCE_LOCKS_POSSIBLE() && defined(OS_MACOSX)
   return true;
 #else
   // Security concerns prevent the use of priority inheritance mutexes on Linux.
-  //   * CVE-2010-0622 - wake_futex_pi unlocks incorrect, possible DoS.
+  //   * CVE-2010-0622 - Linux < 2.6.33-rc7, wake_futex_pi possible DoS.
   //     https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2010-0622
   //   * CVE-2012-6647 - Linux < 3.5.1, futex_wait_requeue_pi possible DoS.
   //     https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2012-6647
@@ -92,7 +96,7 @@ bool LockImpl::PriorityInheritanceAvailable() {
   //   * glibc Bug 14652: https://sourceware.org/bugzilla/show_bug.cgi?id=14652
   //     Fixed in glibc 2.17.
   //     Priority inheritance mutexes may deadlock with condition variables
-  //     during recacquisition of the mutex after the condition variable is
+  //     during reacquisition of the mutex after the condition variable is
   //     signalled.
   return false;
 #endif

base/task_scheduler/scheduler_single_thread_task_runner_manager_unittest.cc

@@ -246,10 +246,12 @@ TEST_P(TaskSchedulerSingleThreadTaskRunnerManagerCommonTest,
   waitable_event_background.Wait();
   waitable_event_normal.Wait();
 
-  if (Lock::HandlesMultipleThreadPriorities())
+  if (Lock::HandlesMultipleThreadPriorities() &&
+      PlatformThread::CanIncreaseCurrentThreadPriority()) {
     EXPECT_EQ(ThreadPriority::BACKGROUND, thread_priority_background);
-  else
+  } else {
     EXPECT_EQ(ThreadPriority::NORMAL, thread_priority_background);
+  }
   EXPECT_EQ(ThreadPriority::NORMAL, thread_priority_normal);
 }