[llvm-dev] call_once and TSan

[Dmitry Vyukov via llvm-dev]

On Fri, Sep 2, 2016 at 12:09 PM, Kuba Brecka <kuba.brecka at gmail.com> wrote:
>
>> On 2 Sep 2016, at 11:18, Dmitry Vyukov via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>
>> On Thu, Sep 1, 2016 at 2:30 PM, Kuba Brecka <kuba.brecka at gmail.com> wrote:
>>> Hi,
>>>
>>> I'm trying to write a TSan interceptor for the C++11 call_once function.  There are currently false positive reports, because the inner __call_once function is located in the (non-instrumented) libcxx library, and on macOS we can't expect the users to build their own instrumented libcxx.
>>>
>>> TSan already supports pthread_once and dispatch_once by having interceptors that re-implement the logic.  However, doing the same for call_once/__call_once doesn't work, because call_once is explicitly supposed to be exception-safe, but the sanitizer runtime libraries disallow exception handling.
>>>
>>> Any ideas how to handle call_once in TSan?
>>
>> Does anybody remember exact reasons we disable exceptions in sanitizer
>> runtimes? One is that it won't link with C programs. Are there any
>> other?
>> If C is the only reason: there is already a part of tsan runtime is
>> linked only to C++ programs (it contains operator new/delete
>> interceptors). We could add additional files to the cxx part of
>> runtime and build them with exceptions.
>>
>> Alternatively, the interceptor can handle only synchronization but
>> forward actual logic to the real function. Along the lines of:
>>
>> INTERCEPTOR(call_once, o) {
>>  __tsan_acquire_release(o);
>>  REAL(call_once)(o);
>> }
>>
>> That will have some performance impact. If we hardcode the "fully
>> initialized" value, then we can eliminate the additional overhead:
>>
>> INTERCEPTOR(call_once, o) {
>>  if (__atomic_load(o, acquire) == FULLY_INITIALIZED) {
>>     __tsan_acquire(o);
>>     return;
>>  }
>>  __tsan_acquire_release(o);
>>  REAL(call_once)(o);
>> }
>
> Unfortunately, the first fast-path check is inlined and cannot be intercepted.  We can only intercept the inner call to __call_once.  But how would __tsan_acquire_release help here?  The issue is that we need to perform the release *after* user code has run, but before the "o" flag is changed.  Otherwise, TSan will still see a false positive where one thread has already run user code, and another thread already sees that call_once is finished, but the acquire has no release to pair with.



Will then the following work?

INTERCEPTOR(call_once, o) {
  REAL(call_once)(o);
  __tsan_release_merge(o);
  __tsan_acquire(o);
}