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Subject: Re: [boost] [lockfree::fifo] Review
From: Helge Bahmann (hcb_at_[hidden])
Date: 2009-12-11 09:47:11

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;

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.

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.


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