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Subject: Re: [boost] [lockfree::fifo] Review
From: Tim Blechmann (tim_at_[hidden])
Date: 2009-12-09 15:49:21

> Yes. Recently I did an experiment on 32 bit 2.6 linux using SUSE 9.1 and
> libc that came with it. (libc version was 2.3.3-97). I was doing all
> sorts of allocations and looking at the returned pointer. What I found
> was that the only the two low bits were always the same (obviously
> because malloc has to return aligned data). All other bits could be
> different depending on the size of the requested allocation. I think
> they were also depended on the thread that was requesting the
> allocation, but I'm not sure on this point. So, other than the 2 lower
> bits, you can't assume you can use any more. And 2 bits seems kinda low
> for an ABA tag.

all 32 bit platforms that i know of are using the full 32bit as address
space (the lower two bits are 0 because of 32bit alignment) ... on 64bit
platforms it is quite different ... ppc64, ia64 and x86_64 all use just
a part of the full address space (iirc 48 bit for x86_64) ... the rest
should be usable as aba prevention tag.

>>> 1. You're missing write barrier after a call to alloc_node().
>>> Rational: alloc_node sets the "next" pointer of the newly allocated node
>>> to NULL. That is a store to shared memory. Setting next pointer to NULL
>>> has to be observable to other threads BEFORE they observe the inserted
>>> node. If you don't insert a write barrier, it is possible for other
>>> threads to see the new element in the list before element's next pointer
>>> is initialized with NULL.
>> i was assuming, that cas issues a full memory barrier (at least my cas
>> implementation does), so i don't think, this would be an issue.
> You know, it's an interesting question whether CAS really implies a
> memory barrier or not. Obviously, you've implemented it that way, but I
> think it not need be that way. I think there's really no "standard" on
> CAS so we need to go with what is commonly accepted. It may be another
> question that would be good to ask at c.p.t
> If someone can answer this question, please chime in.

the c++0x atomic library handles it explicitly ... compare_exchange has
takes the behavior as argument ...

>> i am currently porting the data structures to the boost.atomic proposal,
>> which provides better language support to specify the memory order
>> constraints. the code is available from a separate branch of my
>> repository. i would be curious, if you could have a look at that
>> implementation as well.
> Definitely.
> How can I get to your latest code?

the code can be found in my git repository [1], branch lockfree-atomic
... unfortunately, i am stuck with the fifo implementation, which is
currently broken, although i cannot see why :/

thnx, tim


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