Re: [PATCH v5 05/10] seqlock: Better document raw_write_seqcount_latch()

From: Paul E. McKenney
Date: Mon Apr 13 2015 - 14:43:06 EST

On Mon, Apr 13, 2015 at 11:21:46AM -0700, Linus Torvalds wrote:
> On Mon, Apr 13, 2015 at 10:43 AM, Paul E. McKenney
> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> >
> > A shorthand for READ_ONCE + smp_read_barrier_depends() is the shiny
> > new lockless_dereference()
> Related side note - I think people should get used to seeing
> "smp_load_acquire()". It has well-defined memory ordering properties
> and should generally perform well on most architectures. It's (much)
> stronger than lockless_dereference(), and together with
> smp_store_release() you can make rather clear guarantees about passing
> data locklessly from one CPU to another.
> I'd like to see us use more of the pattern of
> - one thread does:
> .. allocate/create some data
> smp_store_release() to "expose it"
> - another thread does:
> smp_load_acquire() to read index/pointer/flag/whatever
> .. use the data any damn way you want ..
> and we should probably aim to prefer that pattern over a lot of our
> traditional memory barriers.

I couldn't agree more!

RCU made a similar move from open-coding smp_read_barrier_depends()
to using rcu_dereference() many years ago, and that change made RCU
code -much- easier to read and understand. I believe that moving
from smp_mb(), smp_rmb(), and smp_wmb() to smp_store_release() and
smp_load_acquire() will provide similar maintainability benefits.
Furthermore, when the current code uses smp_mb(), smp_store_release() and
smp_load_acquire() generate faster code on most architectures.

Thanx, Paul

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