[fpc-devel] Why FreeBSD sem_init() works different to Linux?

[Ewald]

Once upon a time, on 02/08/2013 04:02 PM to be precise, Jonas Maebe said:
>
> On 07 Feb 2013, at 16:52, Ewald wrote:
>
>> Altough I still
>> don't see how you can make these calls atomic then (without the
>> barrier)?
>
> On x86, it is impossible to perform an atomic access without a
> (partial) memory barrier that affects all memory (due to the behaviour
> of the "lock" prefix). On other architectures, the instructions for
> atomic accesses may only lock the cacheline or memory page containing
> the value to be atomically modified. I.e., they are only a barrier for
> that cacheline/page, not for the entire memory.
>
> Or not even that: older SPARC architectures only had a test-and-set
> instruction, which only supported switching between 0 and 1. So
> basically you had to use a single global lock variable, which you
> locked using this instruction, then you modified the "atomic value"
> using regular instructions, and then unlocked the global lock again.
> So any atomic update conflicted with any other atomic update,
> regardless of where the values were located, and there was no memory
> barrier whatsoever (except for this one lock variable). And it only
> worked if all code in the entire program made use of the same global
> lock variable. It's similar for older ARM CPUs.
Now I see, thanks for the explanation!

-- 
Ewald

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