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SubjectRe: Behaviour of smp_mb__{before,after}_spin* and acquire/release
On Wed, Jan 14, 2015 at 11:31:47AM +0000, Will Deacon wrote:
> Hi Oleg,
>
> On Tue, Jan 13, 2015 at 06:45:10PM +0000, Oleg Nesterov wrote:
> > 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().
>
> Yup, that makes sense. The comment is consistent with the code, and I think
> the code is doing what it's supposed to do.
>
> > 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().
>
> In which case, it's probably a good idea to document that too.
>
> > As for "prior loads", this doesn't look true...
>
> Agreed. I'd propose something like the diff below, but it also depends on
> my second question since none of this is true for smp_load_acquire.

OK, finally getting to this, apologies for the delay...

It does look like I was momentarily confusing the memory ordering implied
by lock acquisition with that by smp_lock_acquire(). Your patch looks good,
would you be willing to resend with commit log and Signed-off-by?

Thanx, Paul

> Will
>
> --->8
>
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index 70a09f8a0383..9c0e3c45a807 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -1724,10 +1724,9 @@ for each construct. These operations all imply certain barriers:
>
> Memory operations issued before the ACQUIRE may be completed after
> the ACQUIRE operation has completed. An smp_mb__before_spinlock(),
> - combined with a following ACQUIRE, orders prior loads against
> - subsequent loads and stores and also orders prior stores against
> - subsequent stores. Note that this is weaker than smp_mb()! The
> - smp_mb__before_spinlock() primitive is free on many architectures.
> + combined with a following ACQUIRE, orders prior stores against
> + subsequent loads and stores. Note that this is weaker than smp_mb()!
> + The smp_mb__before_spinlock() primitive is free on many architectures.
>
> (2) RELEASE operation implication:
>
>



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