Re: [boost] Notice: Boost.Atomic (atomic operations library)

Subject: Re: [boost] Notice: Boost.Atomic (atomic operations library)
From: Helge Bahmann (hcb_at_[hidden])
Date: 2009-12-05 12:52:38

On Fri, 4 Dec 2009, Phil Endecott wrote:
> Helge Bahmann wrote:
>> there is a pseudo-code explanation in the boost::atomic class documentation
>
> Ah sorry, I didn't realise that compare_exchange_* were in the external 
> interface; for some reason I thought they were internal.

they are external, but I have now changed the implementation that backends 
must implement "four-operand" compare_exchange_*, which specifies two 
memory_order operands (one for successful exchange, one for failed), but 
give it a day until it is committed into the public repo

the three-operand version is then provided by the front-end

>
> OK, so since my kernel_cmpxchg() function doesn't return the old value but 
> only a flag, I need something like:
>
> bool success = kernel_cmpxchg(e,d,&i);
> if (!success) e = i;
>
> But i may have changed again between the kernel_cmpxchg() and that 
> assignment.  Is that OK?

that's okay -- there are two cases:
- "new" value (loaded after failed cmpxchg) does not match "expected", 
then the caller receive a more up-to-date value and retry with this one, 
after all it does not matter whether it failed with the "old" or "new" 
mismatching value

- "new" value (loaded after failed cmpxchg) matches "expected": 
compare_exchange_weak is allowed to fail "spuriously"

> Should I use load() there?  If so, what memory order is needed?

the load is internal, so load it "relaxed" , you only have to maintain 
the memory order for the "total" cmpxchg, successful or failed

Best regards,
Helge