Re: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking cycles

[Manfred Spraul]

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
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