Re: [Boost-users] [weak_ptr] thread safe?

From: Peter Dimov (pdimov_at_[hidden])
Date: 2006-04-18 15:43:40

Alain Cormier wrote:
> Hello group!
>
> We are experiencing problems with boost::weak_ptr in a multi-threaded
> environment. It looks that the lock on the ref counting doesn't work
> well with weak_ptr in release build.
>
> configuration :
> Windows 2000 SP4
> VC6 SP5
> boost 1.33.1
>
> In summary, it seems that add_ref_lock in sp_counted_base_w32.hpp has
> a bug in release. In disassemblies, we observe :
>
> 01   lea         esi,[eax+4]
> 02   mov         eax,dword ptr [esi]
> 03   test        eax,eax
> 04   je          TestWeakPtr+0E4h (00401244)
> 05   lea         ecx,[eax+1]
> 06   mov         edx,esi
> 07   lock cmpxchg dword ptr [edx],ecx
> 08   mov         ecx,eax
> 09   cmp         ecx,eax
> 10   je          TestWeakPtr+7Dh (004011dd)
>
> At line 08, you'll see that we move eax into ecx and after (line 09)
> we compare ecx and eax that are obviously the same which will destruct
> prematurely our pointer. In attachement, I send a complete program
> reproducing the bug with VC6 in release.
>
> It is probably a bug in VC6. But is it caused by a bad use? is it
> simply a "bug" in boost 1.33.1?

It seems that the VC6 optimizer doesn't know that the 
InterlockedCompareExchange intrinsic:

> 07   lock cmpxchg dword ptr [edx],ecx
> 08   mov         ecx,eax

destroys eax.

I'm not sure how to fix this reliably without using a .cpp file.