libc: provide atomic operations will full barriers for NDK apps.
__atomic_cmpxchg and other related atomic operations did not
provide memory barriers, which can be a problem for non-platform
code that links against them when it runs on multi-core devices.
This patch does two things to fix this:
- It modifies the existing implementation of the functions
that are exported by the C library to always provide
full memory barriers. We need to keep them exported by
the C library to prevent breaking existing application
machine code.
- It also modifies <sys/atomics.h> to only export
always-inlined versions of the functions, to ensure that
any application code compiled against the new header will
not rely on the platform version of the functions.
This ensure that said machine code will run properly on
all multi-core devices.
This is based on the GCC built-in sync primitives.
The end result should be only slightly slower than the
previous implementation.
Note that the platform code does not use these functions
at all. A previous patch completely removed their usage in
the pthread and libstdc++ code.
+ rename arch-arm/bionic/atomics_arm.S to futex_arm.S
+ rename arch-x86/bionic/atomics_x86.S to futex_x86.S
+ remove arch-x86/include/sys/atomics.h which already
provided inlined functions to the x86 platform.
Change-Id: I752a594475090cf37fa926bb38209c2175dda539
diff --git a/libc/include/sys/atomics.h b/libc/include/sys/atomics.h
index d3fa145..3ada8de 100644
--- a/libc/include/sys/atomics.h
+++ b/libc/include/sys/atomics.h
@@ -33,10 +33,48 @@
__BEGIN_DECLS
-extern int __atomic_cmpxchg(int old, int _new, volatile int *ptr);
-extern int __atomic_swap(int _new, volatile int *ptr);
-extern int __atomic_dec(volatile int *ptr);
-extern int __atomic_inc(volatile int *ptr);
+/* Note: atomic operations that were exported by the C library didn't
+ * provide any memory barriers, which created potential issues on
+ * multi-core devices. We now define them as inlined calls to
+ * GCC sync builtins, which always provide a full barrier.
+ *
+ * NOTE: The C library still exports atomic functions by the same
+ * name to ensure ABI stability for existing NDK machine code.
+ *
+ * If you are an NDK developer, we encourage you to rebuild your
+ * unmodified sources against this header as soon as possible.
+ */
+#define __ATOMIC_INLINE__ static __inline__ __attribute__((always_inline))
+
+__ATOMIC_INLINE__ int
+__atomic_cmpxchg(int old, int _new, volatile int *ptr)
+{
+ /* We must return 0 on success */
+ return __sync_val_compare_and_swap(ptr, old, _new) != old;
+}
+
+__ATOMIC_INLINE__ int
+__atomic_swap(int _new, volatile int *ptr)
+{
+ int prev;
+ do {
+ prev = *ptr;
+ } while (__sync_val_compare_and_swap(ptr, prev, _new) != prev);
+ return prev;
+}
+
+__ATOMIC_INLINE__ int
+__atomic_dec(volatile int *ptr)
+{
+ return __sync_fetch_and_sub (ptr, 1);
+}
+
+__ATOMIC_INLINE__ int
+__atomic_inc(volatile int *ptr)
+{
+ return __sync_fetch_and_add (ptr, 1);
+}
+
int __futex_wait(volatile void *ftx, int val, const struct timespec *timeout);
int __futex_wake(volatile void *ftx, int count);