[LLVMdev] RFC: Atomics.h

Owen Anderson resistor at mac.com
Sun May 17 12:08:45 CDT 2009


On May 17, 2009, at 5:23 AM, Luke Dalessandro wrote:

> Owen Anderson wrote:
>>
>> On May 16, 2009, at 7:47 PM, Luke Dalessandro wrote:
>>
>>> Also, atomic ops are usually pretty low level things used for
>>> nonblocking algorithms or to build higher level locking  
>>> constructs. Is
>>> that the plan here too? It seems like you'd want to avoid anything  
>>> too
>>> fancy since LLVM has to run on so many different architectures with
>>> their variety of memory semantics, etc.
>>
>> I totally agree.  However, at least one case of thread-unsafety
>> (ManagedStatic), has proven very-difficult-to-impossible to implement
>> correctly without using lower-level operations.
>>
>
> Yes, double-checked locking is a pain. There's a C++ safe  
> implementation
> in
> http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html
> in the "Making it work with explicit memory barriers" section. As  
> far as
> I know, it is still considered to work.

Our problems are actually deeper than that, because we need to  
interact well with static constructors.  This means that we can't use  
a mutex with a non-constant initializer, or else we can't depend on it  
being properly initialized before the ManagedStatic is accessed.   
While this would be possible with pthread mutexes, I know of no good  
way to do it for Windows CRITICAL_SECTION's.

--Owen
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