Subj : Re: double-checked locking in C
To   : comp.programming.threads
From : Chris Friesen
Date : Thu Jul 07 2005 05:16 pm

Markus Elfring wrote:
>>You're mistaken; processors can reorder memory accesses across jumps.
> 
> 
> Would anybody like to point to the possible reorderings that are described in a CPU
> design manual?
> How much does it have in common with the technique "out-of-order execution"?

We could, but it doesn't matter.

CPUs are allowed to do *ANYTHING* as long as it meets the standard.

Consider a hypothetical cpu that does everything it can to screw up 
programs which are not written to the POSIX standard:  It's a multi-cpu 
box with no cache coherence.  Data is modified in local cache, and 
random bits of it get written to main memory.  When you release the 
lock, then main memory is updated.  Random bits of main memory are read 
into the local cache, but when you aquire an appropriate lock the local 
cache gets updated.

Without assembly language memory barriers, it's impossible for DCL to 
work on such a system.

For portability, code should be written to comply to POSIX, full stop. 
If POSIX doesn't guarantee a behaviour, you can't rely on it.

Chris

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