[llvm-dev] [TSAN] LLVM statistics and pass initialization trigger race detection

[Mehdi Amini via llvm-dev]

Hi,

Thanks for the answers.

> On Apr 18, 2016, at 10:38 AM, Kostya Serebryany <kcc at google.com> wrote:
> 
> +dvyukov
> 
> 
> 
> On Fri, Apr 15, 2016 at 10:24 PM, Mehdi Amini via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> Hello,
> 
> I trying TSAN on Darwin on LLVM itself (sanitizing multi-threaded ThinLTO link).
> However I see two main issues on my debug build: 
> 
> 1) Statistics: the pre/post increment is not safe, it seems to be acknowledge in the code itself:
> 
>     // FIXME: This function and all those that follow carefully use an
>     // atomic operation to update the value safely in the presence of
>     // concurrent accesses, but not to read the return value, so the
>     // return value is not thread safe.
> 
> Can we tell TSAN to ignore the statistics somehow? Alternatively, is there a fix someone can suggest?
> 
> It's better to add proper synchronization, most likely an atomic read with memory_order_relaxed is enough. 

Ok, will look into this!

>  
> 
> 2) Pass initialization. This macro does not please TSAN (even though it seems written with TSAN in mind):
> 
> #define CALL_ONCE_INITIALIZATION(function) \
>   static volatile sys::cas_flag initialized = 0; \
>   sys::cas_flag old_val = sys::CompareAndSwap(&initialized, 1, 0); \
>   if (old_val == 0) { \
>     function(Registry); \
>     sys::MemoryFence(); \
>     TsanIgnoreWritesBegin(); \
>     TsanHappensBefore(&initialized); \
>     initialized = 2; \
>     TsanIgnoreWritesEnd(); \
>   } else { \
>     sys::cas_flag tmp = initialized; \
>     sys::MemoryFence(); \
>     while (tmp != 2) { \
>       tmp = initialized; \
>       sys::MemoryFence(); \
>     } \
>   } \
>   TsanHappensAfter(&initialized);
> 
> 
> This code might have been added to please the old valgrind-based tsan (which did not understand atomics). 
> For the current tsan we should just implement proper synchronization and remove the annotations. 
> Why can't we use std::call_once here? 

Maybe some pre c++11 thing? 
I'll try to upgrade std::call_once.


> 
> Looks like you are serious about running llvm under tsan. Very cool! 
> Does it make sense to add a tsan build to our sanitizer bot <http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-fast> (which already has asan, ubsan, lsan, and msan)? 
> What tests and what build mode would you suggest for the bot? 

I plan on adding a TSan bot on Green Dragon in the near future.
My idea is to perform a ThinLTO bootstrap of clang (i.e. stage 1 being a Release+Assert+TSAN and stage2 Release-ThinLTO). This involves running the optimizer and the codegen in multiple threads.


-- 
Mehdi



> 
> --kcc 
>  
> 
> The failure looks like:
> 
> ==================
> WARNING: ThreadSanitizer: data race (pid=17192)
>   Atomic write of size 4 at 0x0001113619e8 by thread T13:
>     #0 __tsan_atomic32_compare_exchange_val <null>:568340296 (libclang_rt.tsan_osx_dynamic.dylib+0x00000003e7aa)
>     #1 llvm::sys::CompareAndSwap(unsigned int volatile*, unsigned int, unsigned int) Atomic.cpp:52 (libLTO.dylib+0x000000511914)
>     #2 llvm::initializeSimpleInlinerPass(llvm::PassRegistry&) InlineSimple.cpp:82 (libLTO.dylib+0x000000ab2b55)
>     #3 (anonymous namespace)::SimpleInliner::SimpleInliner() InlineSimple.cpp:50 (libLTO.dylib+0x000000ab2e8e)
>     #4 (anonymous namespace)::SimpleInliner::SimpleInliner() InlineSimple.cpp:49 (libLTO.dylib+0x000000ab2d19)
>     #5 llvm::createFunctionInliningPass() InlineSimple.cpp:85 (libLTO.dylib+0x000000ab2ce3)
> ...
> 
>   Previous read of size 4 at 0x0001113619e8 by thread T14:
>     #0 llvm::initializeSimpleInlinerPass(llvm::PassRegistry&) InlineSimple.cpp:82 (libLTO.dylib+0x000000ab2b65)
>     #1 (anonymous namespace)::SimpleInliner::SimpleInliner() InlineSimple.cpp:50 (libLTO.dylib+0x000000ab2e8e)
>     #2 (anonymous namespace)::SimpleInliner::SimpleInliner() InlineSimple.cpp:49 (libLTO.dylib+0x000000ab2d19)
>     #3 llvm::createFunctionInliningPass() InlineSimple.cpp:85 (libLTO.dylib+0x000000ab2ce3)
> ...
>   Location is global 'llvm::initializeSimpleInlinerPass(llvm::PassRegistry&)::initialized' at 0x0001113619e8 (libLTO.dylib+0x0000016389e8)
> 
> 
> Thanks!
> 
> Mehdi
> 
> 
> 
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