Re: [Valgrind-users] helgrind and atomic operations

[David F. <fa...@kd...>]

On Sunday 26 August 2012 01:30:43 David Faure wrote:
> On Saturday 25 August 2012 22:43:58 Julian Seward wrote:
> > Or maybe Qt really is racey
> 
> Bingo :)

OK, maybe not. Turns out the code runs as intended by the Qt developers.

Marc Mutz said "
The standard says it's racy, but the implementation of 
std::atomic::load(memory_order_acquire) won't look different. Simple reads 
and writes on x86 are already sequentially consistent. Think MESI cache 
coherency. Before a CPU writes to a memory location it needs to acquire 
exclusive ownership (E) of the cache line, the well-known "hardware mutex" on 
a cache line that produces False Sharing, too. This seems to hold for all 
architectures, cf. threads re: "Brief example ..." at 
http://www.decadent.org.uk/pipermail/cpp-threads/2008-December/thread.html
"

I attached a pure C++ testcase of the issue. Compile it with
g++ testcase_atomic_ops_helgrind.cpp -o testcase_atomic_ops_helgrind -lpthread

Thiago expects that helgrind can't autodetect this case and that helgrind-
macros markup is needed in Qt, I'm fine with adding that if you guys agree -- 
after you show me how by modifying the attached example :-)

Thanks.

-- 
David Faure, fa...@kd..., http://www.davidfaure.fr
Sponsored by Nokia to work on KDE, incl. KDE Frameworks 5