topic About lockfree_mpmc ... in Intel® Moderncode for Parallel Architectures

About lockfree_mpmc ...

aminer10 — Sun, 22 Jul 2012 22:25:24 GMT

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 architectureLoads 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 tolasttail.

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 thinki have to insert a load between

line [1] and line [2].

What do you think ?

Sincerely,

Amine Moulay Ramdane.

About lockfree_mpmc ...

aminer10 — Sun, 22 Jul 2012 22:35:40 GMT

I wrote:

> 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].

Even if i insert a load beween line [1] and [2] this will not work ,

i have to insert an mfence between line [1] and [2].

What do you think ?

Sincerely,

Amine Moulay Ramdane.

About lockfree_mpmc ...

aminer10 — Sun, 22 Jul 2012 22:53:01 GMT

Hello,

I have inserted an mfence between line [1] and [2] in lockfree_mpmc andi have

updated all my programs that uses lockfree_mpmc,and you can download all

the source codes of my programs from:

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

Sincerely,

Amine Moulay Ramdane.