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

From: Alain Cormier (alain.work_at_[hidden])
Date: 2006-04-19 09:40:46

So, can I conclude that it is a bad behavior in boost::weak_ptr? For now, I
have simply changed add_ref_lock to :

long tmp2 = tmp + 1;
if(InterlockedCompareExchange ( &use_count_, tmp2, tmp ) == tmp2 - 1 )
return true;

This way, VC6 doesn't optimise the return value of
InterlockedCompareExchange and the disassembly looks ok. Can I expect this
workaround (or any better) in boost in a near future so that weak_ptr truly
work with VC6 in release and multi-threaded environment?

Thank you,

Alain

On 4/18/06, Peter Dimov <pdimov_at_[hidden]> wrote:
>
> 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.
>
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