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Subject: [Bug sanitizer/48076] Unsafe double checked locking in __emutls_get_address
Date: Wed, 28 Nov 2012 03:57:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48076

--- Comment #6 from Dmitry Vyukov <dvyukov at google dot com> 2012-11-28 03:56:50 UTC ---
There seems to be a similar bug in code generated for function static
variables.
The fast-path load is a plain load rather than atomic acquire load.

On Sat, Nov 24, 2012 at 3:06 PM, Dmitry Vyukov <dvyukov@google.com> wrote:
> On Fri, Nov 23, 2012 at 8:39 PM, Jakub Jelinek <jakub@redhat.com> wrote:
>> On Fri, Nov 23, 2012 at 08:10:39PM +0400, Dmitry Vyukov wrote:
>>> That's what llvm does as well. But it inserts a fast path before
>>> __cxa_guard_acquire -- acquire-load of the lock word. Doesn't gcc do the
>>> same?
>>
>> Yes, except it isn't __atomic_load_*, but plain memory read.
>>   _3 = MEM[(char *)&_ZGVZ3foovE1a];
>>   if (_3 == 0)
>>     goto <bb 3>;
>>   else
>>     goto <bb 8>;
>>
>>   <bb 8>:
>>   fast path, whatever;
>>
>>   <bb 3>:
>>   _5 = __cxa_guard_acquire (&_ZGVZ3foovE1a);
>>   ...
>>
>> So, right now tsan would just instrument it as __tsan_read1 from
>> &_ZGVZ3foovE1a rather than any atomic load.
>
>
> Looks like a bug. That needs to be load-acquire with proper compiler
> and hardware memory ordering.