[cpp-threads] [Javamemorymodel-discussion] there's a happens-before orderhere. right?

[Anthony Williams]

At Sat 13 Dec 2008 14:42:43 UTC, Alexander Terekhov  
<alexander.terekhov at gmail.com> wrote:

> Let's take your first example and slightly change it by renaming y
> to "data" and using std::atomic<> for x:
>
>     int data; //= 0
>     atomic<int> x; //= 0
>
>     thread 1:
>     ------------
>     if (x.load(relaxed) > 0)
>       data = 1;
>
>     thread 2:
>     ------------
>     data = 2;
>
> This is still data-race-free, right? I hope so.

There's no writes to x, so the load must always read 0 => no race.

> Now let's introduce a store to std::atomic<> x in thread 2.
>
>     int data; //= 0
>     atomic<int> x; //= 0
>
>     thread 1:
>     ------------
>     if (x.load(relaxed) > 0)
>       data = 1;
>
>     thread 2:
>     ------------
>     data = 2;
>     x.store(-1, release);
>
> I just can't conceive how that could possibly introduce a race on
> "data"...

Agreed. The load may read 0 (initial value) or -1 (stored value), but  
in neither case is the branch followed, so there is no race.

> The next step is to change x.store(-1, release) to x.store(+1,
> release) :
>
>     int data; //= 0
>     atomic<int> x; //= 0
>
>     thread 1:
>     ------------
>     if (x.load(relaxed) > 0)
>       data = 1;
>
>     thread 2:
>     ------------
>     data = 2;
>     x.store(+1, release);
>
> This just ought to be data-race-free as well!!!

I don't see why. The load is relaxed, so it is unordered. If it reads  
"+1", there is still no happens-before relationship between the two  
stores to data, so we have a race.

If you need the ordering, use memory_order_acquire for the load.  
That's what it's there for. If there was a data dependency you could  
use memory_order_consume instead.

It is unfortunate that the C++0x atomics don't neatly map to the most  
efficient instruction sequence on any particular architecture, but not  
all current (and future) architectures have the same set of  
constraints and guarantees.

Anthony
-- 
Anthony Williams
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