Subject: Re: [v3,11/41] mips: reuse asm-generic/barrier.h

From: Peter Zijlstra <[email-masked]>
Date: Tue, 26 Jan 2016 11:24:02 +0100

On Thu, Jan 14, 2016 at 02:20:46PM -0800, Paul E. McKenney wrote:
> On Thu, Jan 14, 2016 at 01:24:34PM -0800, Leonid Yegoshin wrote:
> > On 01/14/2016 12:48 PM, Paul E. McKenney wrote:
> > >
> > >So SYNC_RMB is intended to implement smp_rmb(), correct?
> > Yes.
> > >
> > >You could use SYNC_ACQUIRE() to implement read_barrier_depends() and
> > >smp_read_barrier_depends(), but SYNC_RMB probably does not suffice.
> > 
> > If smp_read_barrier_depends() is used to separate not only two reads
> > but read pointer and WRITE basing on that pointer (example below) -
> > yes. I just doesn't see any example of this in famous
> > Documentation/memory-barriers.txt and had no chance to know what you
> > use it in this way too.
> 
> Well, Documentation/memory-barriers.txt was intended as a guide for Linux
> kernel hackers, and not for hardware architects.

Yeah, this goes under the header: memory-barriers.txt is _NOT_ a
specification (I seem to keep repeating this).

> ------------------------------------------------------------------------
> 
> commit 955720966e216b00613fcf60188d507c103f0e80
> Author: Paul E. McKenney <[email-masked]>
> Date:   Thu Jan 14 14:17:04 2016 -0800
> 
>     documentation: Subsequent writes ordered by rcu_dereference()
>     
>     The current memory-barriers.txt does not address the possibility of
>     a write to a dereferenced pointer.  This should be rare, 

How are these rare? Isn't:

	rcu_read_lock()
	obj = rcu_dereference(ptr);
	if (!atomic_inc_not_zero(&obj->ref))
		obj = NULL;
	rcu_read_unlock();

a _very_ common thing to do?