Re: [boost] [lockfree::fifo] Review

Subject: Re: [boost] [lockfree::fifo] Review
From: Andy Venikov (avenikov_at_[hidden])
Date: 2009-12-11 10:06:52

Helge Bahmann wrote:
> On Fri, 11 Dec 2009, Tim Blechmann wrote:
> 
>> by definition ll/sc are immune to aba problems
> 
> This is NOT correct. The sequence of operations you would need to 
> prevent ABA is:
> 
>     object *tmp=head.load_linked();
>     object *next=tmp->next;
>     head.store_conditional(next);
> 
> This is however NOT possible because you are not allowed to make any 
> memory access between ll and sc -- some CPUs (definitely older mips, but 
> not ppc, unsure about many of the other architectures) clear the 
> reservation on _any_ memory reference and will cause this sequence to 
> always fail. Alpha has the fun requirement of a memory barrier before 
> you are allowed to dereference "tmp", which just provides another way to 
> kill your reservation.
PPC as well. 7447, for example, will invalidate the "reservation bit" 
after writing to ANY memory location.

> 
> ll/sc only prevents "pathological" ABA races as you cannot inpsect the 
> object being replaced. Exposing an ll/sc interface to a higher-level 
> language is therefore unlikely to result in something usable.
> 
>> most cas-architectures provide dcas
> 
> I think x86 is the rare exception, not the rule, so relying on DCAS 
> excludes pretty much all 32-bit except for x86.
> 
> Helge
> _______________________________________________
> Unsubscribe & other changes: 
> http://lists.boost.org/mailman/listinfo.cgi/boost
>