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About lockfree_mpmc ...

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Hello all,

I think i have discovered a problem with lockfree_mpmc:

You will find the source code of lockfree_mpmc at:

http://pages.videotron.com/aminer/


So please follow with me:

If you take a look at the lockfree_mpmc , here is  the source code
of the push() method:

---

function TLockfree_MPMC.push(tm : tNodeQueue):boolean;
var lasttail,newtemp:long;
i,j:integer;
begin

 if getlength >= fsize
  then
      begin
          result:=false;
          exit;
      end;
result:=true;

newTemp:=LockedIncLong(temp);

[1] lastTail:=newTemp-1;
[2] setObject(lastTail,tm);

repeat

 if CAS(tail,lasttail,newtemp)
   then
      begin
       exit;
      end;
sleep(0);
 until false;

end;

---

As you know in the x86 architecture Loads may be reordered with older stores
to different locations.

So in line [2] there is a load of tail and lasttail to the registers of the 
processor
before calling setobject() and just before on line [1] there is a store to 
lasttail.

So as you have noticed the processor can then reorder the loads on line [2] 
with the older
store on line [1] and this will cause a problem, so i think i have to insert 
a load between
line [1] and line [2].


What do you think ?



Sincerely,
Amine Moulay Ramdane.

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About lockfree_mpmc ... "aminer" <aminer@videotron.ca> - 2012-07-22 18:22 -0500

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