Hi Oleg,
my example was bad, let's continue with your example.
And: If sem_lock() needs another smp_xmb(), then we must add it:
Some apps do not have a user space hot path, i.e. it seems that on some
setups, we have millions of calls per second.
If there is a race, then it will happen.
I've tried to merge your example:
>
> int X = 0, Y = 0;
>
> void func(void)
> {
> bool ll = rand();
>
> if (ll) {
> spin_lock(&local);
> if (!spin_is_locked(&global))
> goto done;
> spin_unlock(&local);
> }
> ll = false;
> spin_lock(&global);
> spin_unlock_wait(&local);
> done:
> smp_rmb(); <<<<<<<<<<<<<<<
> BUG_ON(X != Y);
>
> ++X; ++Y;
>
> if (ll)
> spin_unlock(&local);
> else
> spin_unlock(&global);
> }
I agree, we need the smp_rmb().
I'll write a patch.
We need the full barrier to serialize STORE's as well, but probably we can
rely on control dependancy and thus we only need rmb().
Do we need a full barrier or not?
I don't manage to create a proper line of reasoning.
--
Manfred
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
