[andrewp@andypo.net: Re: libstdc++/5432: Implementation still not thread-safe on multiprocessor machines]

[Andrew Pollard <andrewp at andypo dot net>]

------- Start of forwarded message -------
Date: Sat, 26 Jan 2002 16:56:52 GMT
From: Andrew Pollard <andrewp@andypo.net>
To: ljrittle@gcc.gnu.org, andrew@andypo.net, gcc-bugs@gcc.gnu.org,
   gcc-prs@gcc.gnu.org, rodrigc@gcc.gnu.org, gcc-gnats@gcc.gnu.org
In-reply-to: <20020124212600.20158.qmail@sources.redhat.com>
	(ljrittle@gcc.gnu.org)
Subject: Re: libstdc++/5432: Implementation still not thread-safe on multiprocessor machines

Sorry to repost... I hope first attempt didn't make it out of my machine..
Haven't seen ay responses.... Hope this can make it in for 3.0.4...

Andrew.

>Synopsis: Implementation still not thread-safe on multiprocessor machines
>
>State-Changed-From-To: analyzed->feedback
>State-Changed-By: ljrittle
>State-Changed-When: Thu Jan 24 13:25:59 2002
>State-Changed-Why:
>    Andrew, there were two minor issues with your patch:
>    (And it took collective thought to figure all this out
>    so don't feel bad. ;-)

:-) Just glad to have helped.
   
>    In general, static _Atomic_word should always be init'd
>    to 0 with C++.  We think g++ follows the init order
>    rules of C which are tighter than C++ but for C++
>    non-zero static values may not be init'd until after main()
>    has run (the only rule says it must be done "before
>    the related block [scope] is entered" which might be after
>    threads were started).
 
I didn't realise this. I had assumed that C++ followed the C rules
for 'basic' types... 
   
>    Secondly:
>    
>    <       if (--_M_references == 0)  // XXX MT
>    ---
>    >       if (__exchange_and_add(&_M_references, -1) == 1)
>    
>    is correct (you had == 0).  I.e. assuming it was correct
>    as written (other than thread-safety) your rewrite had a
>    memory leak.

Interesting. This is actually a difference between the gcc-3.0 branch
and the cvs head. I wrote the patch for gcc-3.0.x, and that branch has
the test

    if (_M_references-- == 0)  // XXX MT

So I was right :-) [ But the branch is probably wrong... :-) ] Benjamin
Kosnik changed the head cvs in late June last year....

>    Other than that, Nathan and I have reviewed the patch;
>    I have tested it and installed it (that report to
>    the libstdc++ mailing list).  To be closed after you
>    confirm MP *-*-linux* system fixed as installed and
>    then I move it to 3.0.X branch.  Thanks, Loren

Brilliant. I can confirm that the gcc cvs head as of today
(saturday 26th) compiled and passed the test program on my dual
processor i686 Redhat7.2 system.

Andrew.
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