Re: [Valgrind-users] helgrind/drd annotations for one statement

[Leif W. <lei...@gm...>]

Rdunlock happens before wrlock. 

Sent from my iPhone

On Dec 5, 2012, at 16:51, David Faure <fa...@kd...> wrote:

> On Wednesday 05 December 2012 16:39:47 Leif Walsh wrote:
>> The important synchronization point isn't the rdunlock, it's the wrlock.
> 
> Well, you need two locks, for a happens-before relationship to be established.
> If you remove the rdlock/rdunlock completely (since it could basically be a 
> no-op for a write operation anyway, as others pointed out earlier), then this 
> will be more clear: this write might never become visible to the other thread.
> 
>> I have a hard time believing that you can take a pthread write lock and then
>> look at a value some other processor wrote before you took the lock and not
>> get that value.
> 
> You say "before", but this assumes a global ordering, which you don't get, 
> when not using atomics or the proper locks.
> Each CPU can have a different notion of "before", without the correct 
> synchronization primitives.
> 
> I recommend reading "C++ Concurrency in action" by Anthony Williams, it taught 
> me a lot on all this all works... definitely not a simple topic.
> 
> -- 
> David Faure, fa...@kd..., http://www.davidfaure.fr
> Working on KDE, in particular KDE Frameworks 5
>