Re: [PATCH v2] barriers: introduce smp_mb__release_acquire and update documentation
From: Peter Zijlstra
Date: Wed Oct 21 2015 - 04:24:51 EST
On Tue, Oct 20, 2015 at 04:34:51PM -0700, Paul E. McKenney wrote:
> There is also the question of whether the barrier forces ordering
> of unrelated stores, everything initially zero and all accesses
> READ_ONCE() or WRITE_ONCE():
>
> P0 P1 P2 P3
> X = 1; Y = 1; r1 = X; r3 = Y;
> some_barrier(); some_barrier();
> r2 = Y; r4 = X;
>
> P2's and P3's ordering could be globally visible without requiring
> P0's and P1's independent stores to be ordered, for example, if you
> used smp_rmb() for some_barrier(). In contrast, if we used smp_mb()
> for barrier, everyone would agree on the order of P0's and P0's stores.
Oh!?
> There are actually a fair number of different combinations of
> aspects of memory ordering. We will need to choose wisely. ;-)
>
> My hope is that the store-ordering gets folded into the globally
> visible transitive level. Especially given that I have not (yet)
> seen any algorithms used in production that relied on the ordering of
> independent stores.
I would hope not, that's quite insane.
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