[arm/Linux] Don't rely on KUSER_HELPERS feature

third_party/WebKit/Source/wtf/Atomics.h

 /*
  * Copyright (C) 2007, 2008, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Justin Haygood (jhaygood@reaktix.com)
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 201 matching lines @@
 #if CPU(X86) || CPU(X86_64)
 // Only compiler barrier is needed.
 #if COMPILER(MSVC)
 // Starting from Visual Studio 2005 compiler guarantees acquire and release
 // semantics for operations on volatile variables. See MSDN entry for
 // MemoryBarrier macro.
 #define MEMORY_BARRIER()
 #else
 #define MEMORY_BARRIER() __asm__ __volatile__("" : : : "memory")
 #endif
-#elif CPU(ARM) && (OS(LINUX) || OS(ANDROID))
+#elif CPU(ARM) && OS(ANDROID)
 // On ARM __sync_synchronize generates dmb which is very expensive on single
 // core devices which don't actually need it. Avoid the cost by calling into
 // kuser_memory_barrier helper.
 inline void memoryBarrier()
 {
     // Note: This is a function call, which is also an implicit compiler barrier.
     typedef void (*KernelMemoryBarrierFunc)();
     ((KernelMemoryBarrierFunc)0xffff0fa0)();
 }
 #define MEMORY_BARRIER() memoryBarrier()
@@ skipping 134 matching lines @@
 using WTF::noBarrierLoad;
 using WTF::noBarrierStore;
 
 // These methods allow loading from and storing to poisoned memory. Only
 // use these methods if you know what you are doing since they will
 // silence use-after-poison errors from ASan.
 using WTF::asanUnsafeAcquireLoad;
 using WTF::asanUnsafeReleaseStore;
 
 #endif // Atomics_h