Subject: Re: [PATCH tip/core/rcu 21/21] drivers/vhost: Remove now-redundant read_barrier_depends()

From: "Paul E. McKenney" <[email-masked]>
Date: Tue, 5 Dec 2017 15:39:55 -0800

On Wed, Dec 06, 2017 at 12:09:36AM +0200, Michael S. Tsirkin wrote:
> On Tue, Dec 05, 2017 at 10:57:00PM +0100, Peter Zijlstra wrote:
> > On Tue, Dec 05, 2017 at 11:24:49PM +0200, Michael S. Tsirkin wrote:
> > > READ_ONCE is really all over the place (some code literally replaced all
> > > memory accesses with READ/WRITE ONCE).
> > 
> > Yeah, so?
> 
> Oh my point was I can't just look for READ_ONCE and go
> *that's the pair*. there are too many of these.
> At Paul's suggestion I will document the pairing *this read once has a
> barrier that is paired with that barrier*.
> 
> > Complain to the compiler people for forcing us into that.
> 
> In some cases when you end up with all accesses
> going through read/write once volatile just might better.

That is in fact what the jiffies counter does.  But you lose READ_ONCE()'s
automatic handling of DEC Alpha when you take that approach.

> > > Would an API like WRITE_POINTER()/smp_store_pointer make sense,
> > > and READ_POINTER for symmetry?
> > 
> > No, the whole point of the exercise was to get away from the fact that
> > dependent loads are special.
> 
> It's a pity that dependent stores are still special.

We can make READ_ONCE() not be special at zero cost on non-Alpha
systems, but both smp_wmb() and smp_store_release() are decidedly
not free of added overhead.

							Thanx, Paul