shared_ptr SEGV in Android debug builds

[ian@…]

Hi,

I get a SEGV in the shared_ptr ref counting code on Android debug builds. Release builds do not have this problem.

Instructions:

1. Install ndk (I used r9d 64-bit mac)
2. Ensure ndk-build is on your path
3. Download the attached sample program
4. Run build.sh
5. Install the executable on an Android device (I used a Nexus 7).

As provided, you should get a SEGV in the sp_counted_base code.

There are two ways to fix this:

1. Remove APP_OPTIM=debug from build.sh
2. Enable # LOCAL_CPPFLAGS := -DBOOST_SP_USE_PTHREADS in Android.mk

Notes:

Android debug builds disable the -mthumb option and instead use -marm. Ultimately this affects spinlock.hpp

#elif defined(__GNUC__) && defined( __arm__ ) && !defined( __thumb__ )
#  include <boost/smart_ptr/detail/spinlock_gcc_arm.hpp>
 
//code omitted
 
#elif defined(BOOST_HAS_PTHREADS)
#  include <boost/smart_ptr/detail/spinlock_pt.hpp>

So release builds use spinlock_pt.hpp and debug builds use spinlock_gcc_arm.hpp

[anonymous]

sample app w/ shared_ptr SEGV

[Peter Dimov]

What __ARM_ARCH_*__ macro is defined on the Nexus 7?

[ian@…]

The Android NDK supports compiling for two different arm abis. The default is ARMv5TE and it also supports ARMv7-A. These are configured through the armeabi and armeabi-v7a values, respectively.

It is my understanding that armeabi is a subset of armeabi-v7a so that a binary compiled for armeabi should work on all arm android devices.

I did a quick test to answer your question. I grabbed all the ARM_ARCH macros checked in smart_ptr/detail/spinlock_gcc_arm.hpp (these where the only references I could find in the sample app I posted) and created the following function:

void arm_arch() {

#if defined(ARM_ARCH_7)

const char* arch="defined(ARM_ARCH_7)";

#endif #if defined(ARM_ARCH_7A)

const char* arch="defined(ARM_ARCH_7A)";

#endif #if defined(ARM_ARCH_7R)

const char* arch="defined(ARM_ARCH_7R)";

#endif #if defined(ARM_ARCH_7M)

const char* arch="defined(ARM_ARCH_7M)";

#endif #if defined(ARM_ARCH_7EM)

const char* arch="defined(ARM_ARCH_7EM)";

#endif #if defined(ARM_ARCH_7S)

const char* arch="defined(ARM_ARCH_7S)";

#endif #if defined(ARM_ARCH_6)

const char* arch="defined(ARM_ARCH_6)";

#endif #if defined(ARM_ARCH_6J)

const char* arch="defined(ARM_ARCH_6J)";

#endif #if defined(ARM_ARCH_6K)

const char* arch="defined(ARM_ARCH_6K)";

#endif #if defined(ARM_ARCH_6Z)

const char* arch="defined(ARM_ARCH_6Z)";

#endif #if defined(ARM_ARCH_6ZK)

const char* arch="defined(ARM_ARCH_6ZK)";

#endif #if defined(ARM_ARCH_6T2)

const char* arch="defined(ARM_ARCH_6T2)";

#endif

}

As you might expect, none are defined when compiled for armeabi. When compiled for armeabi-v7a, ARM_ARCH_7A is defined.

Also, the armeabi binary exhibits the crash on the Nexus 7, while the armeabi-v7a does not.

[Andrey Semashev]

Could you also attach the output of command g++ <flags> -dM -E - < /dev/null where <flags> are the compiler switches used by NDK to enable ARMv5TE target?

[Peter Dimov]

Also, the armeabi binary exhibits the crash on the Nexus 7, while the armeabi-v7a does not.

The problem is that, while the ARMv7 and ARMv6 instruction sets have support for memory barriers (the former via a dedicated instruction, DMB, the latter by using the coprocessor #15), earlier instruction sets do not. (Presumably pre-ARMv6 chips didn't support multiprocessing.)

Since the Nexus 7 is a multicore device (I suppose), it needs the memory barriers to be present in the code for the spinlock to work properly. But, since the default configuration doesn't define any ARMv6/ARMv7 macro, we have no way of inserting a memory barrier as we have to assume an earlier instruction set.

In principle, one should be able to compile for ARMv6+ while still using the earlier ABI, but I don't know how.

I'm not sure how we can fix that problem on our side. The only option seems to be to disable spinlock_gcc_arm.hpp altogether when not on ARMv6+ and fall back to phtreads.

[ian@…]

arm-linux-androideabi-g++ -MMD -MP -MF ./obj/local/armeabi/objs-debug/shared_ptr_crash/main.o.d -fpic -ffunction-sections -funwind-tables -fstack-protector -no-canonical-prefixes -march=armv5te -mtune=xscale -msoft-float -fno-exceptions -fno-rtti -mthumb -Os -g -DNDEBUG -fomit-frame-pointer -fno-strict-aliasing -finline-limit=64 -O0 -UNDEBUG -marm -fno-omit-frame-pointer -I/Users/iwhoyt/projects/speedtestnet-android/toolchain/ndk/sources/cxx-stl/gnu-libstdc++/4.6/include -I/Users/iwhoyt/projects/speedtestnet-android/toolchain/ndk/sources/cxx-stl/gnu-libstdc++/4.6/libs/armeabi/include -I/Users/iwhoyt/projects/speedtestnet-android/toolchain/ndk/sources/cxx-stl/gnu-libstdc++/4.6/include/backward -Ijni -DANDROID -Wa,--noexecstack -Wformat -Werror=format-security -frtti -fexceptions -std=gnu++11 -I/Users/iwhoyt/projects/speedtestnet-android/toolchain/ndk/platforms/android-8/arch-arm/usr/include -dM -E - < /dev/null

cc1: warning: command line option '-std=gnu++11' is valid for C++/ObjC++ but not for C [enabled by default]
cc1: warning: command line option '-frtti' is valid for C++/ObjC++ but not for C [enabled by default]
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