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

[Will Deacon]

Hi Paul,

On Tue, Jan 20, 2015 at 03:40:40AM +0000, Paul E. McKenney wrote:
> On Wed, Jan 14, 2015 at 11:31:47AM +0000, Will Deacon wrote:
> > 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...

No problem, it's hardly urgent :)

> 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?

Hey, if you get confused by it then what hope do the rest of us have?

Patch below, thanks.

Will

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