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

[Ewald]

Once upon a time, on 02/07/2013 11:57 AM to be precise, Jonas Maebe said:
>
> On 06 Feb 2013, at 23:16, Ewald wrote:
>
>> Concerning the locking mechanism, you can uses mutex(en/es/ii) or you
>> can do this by a busy waiting loop with an integer (of course there are
>> other possibilities). To elaborate on the latter a bit more:
>>
>> *  TLockType = Integer;
>> *  Initialization: `Lock:= 0;`
>> *  Lock & unlock:
>>
>> ===> Code Begin <===
>>
>> Procedure WaitLockVar(var aLock: Integer);
>> Begin
>>    Repeat
>>    Until InterLockedCompareExchange(aLock, 1, 0) = 0;
>> End;
>>
>> Procedure UnlockVar(var aLock: Integer);
>> Begin
>>    InterlockedExchange(aLock, 0);
>> End;
>>
>> ===> Code End <===
>>
>> This last code is tested and works.
>
> It only works on some platforms (and even there it may change
> depending on which exact processor you are using). InterlockedExchange
> does not guarantee any kind of memory barrier, and hence you will get
> occasional data races on platforms with weakly consistent memory
> models. You have to add memory barriers.
Well, I always thought that the InterLoackedCompareExchange boiles down
to [**]
.Lock CMPXCHG

Or something quite like that. The `.Lock` there is the important part
since this does insure a memory barier. Then the only problem would be
the absence of the keyword `volatile` to ensure the integer is always in
the same location but I've read somewhere, long ago, that fpc doesn't
perform such optimizations.

So I don't see the issue really.

[**] I make this assumption because the documentation says it is done in
a thread safe way (reference:
http://www.freepascal.org/docs-html/rtl/system/interlockedcompareexchange.html),
and I don't see what other things can be made thread safe about this
function except for a memory barrier.

-- 
Ewald

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