Possible errata in the paper as algorithm 4

[trivigy]

@frygge In algorithm 3 line 4 the pointer exchange is done with local temp count being set to 0. However, in algorithm 4 line 4 the exchange is done with local temp count is being set to 1. Looking through the code LINK it seems that the number in the paper should be 0?!

[frygge]

Thanks for the feedback! Let me have look. I'll come back to this as soon as possible.

[frygge]

Maybe there is a misunderstanding, which might be caused by a refactorization of the algorithms to better fit into the paper:

• Algorithm 3 in the paper describes the store function starting in 1215.
• This one first creates a local copy (which increases the local counter in correspondence to line 3 in the paper) and then calls the version with the R-value, which is in line 1217.
• The R-value version of the store function does an exchange (line 1218) on the counted_ptr object, which is not the function in line 1230, but rather 169.
• line 6 in the paper is then done by the destructor of the local copy of the atomic's old value.

I hope, this makes things clearer.

Best, JPS

[trivigy]

I followed that same logic for the store function and in full agreement. Algorithm 3 in the paper is correct. Algorithm 4 is the one where I believe the mistake is.

I assumed the exchange method (line 1230) works almost exactly the same as store (1215), with the one exception that it does not drop the temporary shared_ptr. Compared to the way the store method does (line 1215 after returning from the R-value version).

Therefore, I thought, in the paper algorithm 4 line 4 should say old_cptr = a.cptr.exchange({ 0, s.cptr.pctrl }) but it instead says old_cptr = a.cptr.exchange({ 1, s.cptr.pctrl }). Looking at the code at 1235, when return shared_ptr<T>{ cptr_hdr_.exchange( hdr_ptr_type{ desired_ptr }, order ) }; is called, a new hdr_ptr_type{ desired_ptr } is created. Is that not equivalent to { 0, s.cptr.pctrl }? Which seems to disagree with algo4/line4 and agree with my assumption of the error.

I hope I make sense and that my thinking is sound. Apologies if I am missing a critical detail in my understanding.

[frygge]

My bad. You talked about algorithm 4 in the paper ^^
I'll have a look again asap.

[frygge]

It seems that you are right. Thank you very much!

Indeed, even the proof claims that the exchange function works mostly the same as the store function.

Let me have a look with a sharpened mind after my vacation again. Then I'll also fix the paper. I'll come back to you ;)