1 ANDROID ATOMICS OPERATIONS 2 3 The problem: 4 ------------ 5 6 If your application native code was generated with a NDK release older than r7b 7 and uses any of the following functions defined in the `<sys/atomics.h>` 8 header: 9 10 * `__atomic_cmpxchg` 11 * `__atomic_inc` 12 * `__atomic_dec` 13 * `__atomic_swap` 14 15 Then the corresponding machine code is not guaranteed to work properly on 16 some multi-core Android ARM-based devices (x86 ones are not affected). 17 18 The solution: 19 ------------- 20 21 The `<sys/atomics.h>` header has been updated in NDK r7b. Simply recompiling 22 your _unmodified_ sources with this version of the NDK should be enough to 23 completely eliminate the problem. 24 25 If you can't use NDK r7b or later for some reason, read the section below. 26 27 More details: 28 ------------- 29 30 The main issue is that the implementation of these functions, as provided 31 by the C library, did not provide any associated memory barriers. This is 32 by design, because the platform code that uses them does insert explicit 33 barriers around these operations. 34 35 The functions were only exposed through the NDK by mistake, they were not 36 supposed to be used from applications. Any application code that use them 37 without inserting its own barriers may experiment incorrect behaviour, 38 which can result in bugs that are very hard to reproduce and diagnose. 39 40 Not all multi-core devices are affected though. Certain OEMs enforce a 41 policy where all threads of a single process are forced to run on the same 42 core. In this case, the bug cannot occur, unless you're directly accessing 43 shared memory between two processes. 44 45 The problem is only likely to be seen on devices running Android 3.0 to 46 Android 4.1. The C library implementation in 4.1 has been updated to provide 47 full memory barriers as well. This ensures existing native code keeps 48 working correctly on this platform and future ones, even if they were not 49 recompiled. 50 51 We still strongly recommend recompiling your native code to ensure you'll 52 never have to debug this issue (life is short). In the case where this would 53 not be possible (e.g. you're using an older version of the NDK for some 54 reason, or a custom build system / toolchain), we recommend stopping from 55 using these functions entirely. Very fortunately, GCC provides handy 56 intrinsics functions that work with very reasonable performance and always 57 provide a *full* *barrier*. 58 59 * `__sync_fetch_and_add` instead of `__atomic_inc` 60 * `__sync_fetch_and_sub` instead of `__atomic_sub` 61 * `__sync_val_compare_and_swap` instead of `__atomic_cmpxchg` 62 63 See the content of `platforms/android-3/arch-arm/usr/include/sys/atomics.h` 64 to see how these can be used. 65 66 See the [GCC documentation about __sync_ functions](http://gcc.gnu.org/onlinedocs/gcc/_005f_005fsync-Builtins.html#_005f_005fsync-Builtins) for more information: 67 68
