Re: Behaviour of smp_mb__{before,after}_spin* and acquire/release

[Oleg Nesterov <oleg@redhat.com>]

On 01/13, Will Deacon wrote:
>
>   1. Does smp_mb__before_spinlock actually have to order prior loads
>      against later loads and stores? Documentation/memory-barriers.txt
>      says it does, but that doesn't match the comment

The comment says that smp_mb__before_spinlock() + spin_lock() should
only serialize STOREs with LOADs. This is because it was added to ensure
that the setting of condition can't race with ->state check in ttwu().

But since we use wmb() it obviously serializes STOREs with STORES. I do
not know if this should be documented, but we already have another user
which seems to rely on this fact: set_tlb_flush_pending().

As for "prior loads", this doesn't look true...

Oleg.