[llvm-dev] Using TSAN along with custom llvm IR pass

[Dmitry Vyukov via llvm-dev]

On Tue, Jul 11, 2017 at 3:26 PM, Nischai Vinesh
<nischai.vinesh at gmail.com> wrote:
> Hi Dmitry,
>
> I managed to get the program instrumented with tsan using 'opt -tsan ..',
> but still it is not printing tsan warnings when I run the binary. When I
> instrument the same program with tsan during compilation using 'clang
> -fsanitize=thread ...', without instrumenting my IR pass, it does give out
> tsan warnings!
>
>
> On Monday, July 10, 2017 at 7:21:28 AM UTC+2, Dmitry Vyukov wrote:
>>
>> On Sun, Jul 9, 2017 at 3:50 PM, Nischai Vinesh <nischai... at gmail.com>
>> wrote:
>> > Hello,
>> >
>> > I tried that as well but there are no tsan warnings thrown at run time,
>> > even
>> > though there are data race error!
>> >
>> > The steps I followed:
>> > 1. Generate bitcode using 'clang -emit-llvm' command
>> > 2. Using the 'opt -load ..' command, I instrumented the bitcode file
>> > with my
>> > custom pass
>> > 3. Using the command 'opt -tsan ..', instrumented the output bitcode
>> > file
>> > from step 2
>> > 4. Using the llvm backend compiler, 'llc', compiled the program to
>> > native
>> > assembly
>> > 5. The output from step 4 is compiled again with 'clang
>> > -fsanitize=thread'
>> > (linking tsan here) for the final object file.
>> >
>> > Executing this final output file works fine, except that it doesn't
>> > throws
>> > any TSAN warnings!
>>
>>
>> Hi Nischai,
>>
>> I see 3 possibilities:
>> 1. Either the code is somehow ends up being non-instrumented, or
>> 2. tsan does not consider what it sees a data race, or
>> 3. the data race is masked by unrelated synchronization.
>>
>> For 1 you can check that the resulting code in fact contains __tsan_*
>> callbacks.
>> For 2 and 3 you can rebuilt tsan runtime with TSAN_DEBUG_OUTPUT=2
>> define, then it will print everything it sees (memory accesses and
>> synchronization). This output will allow us to figure out why it does
>> not report a race.
>
>
> So I guess we can rule out the 1st possibility. Can you kindly tell me how
> to rebuild tsan runtime? Sorry, I am new to LLVM and stuff.

Here is the script we use on build bots:
https://github.com/llvm-mirror/zorg/blob/master/zorg/buildbot/builders/sanitizers/buildbot_standard.sh

You need this option with -DCOMPILER_RT_DEBUG=ON
-DCOMPILER_RT_TSAN_DEBUG_OUTPUT=ON:

echo @@@BUILD_STEP build tsan with stats and debug output@@@
build_tsan "${TSAN_FULL_DEBUG_BUILD_DIR}" "-DCOMPILER_RT_DEBUG=ON
-DCOMPILER_RT_TSAN_DEBUG_OUTPUT=ON -DLLVM_INCLUDE_TESTS=OFF" gcc g++