From mboxrd@z Thu Jan  1 00:00:00 1970
From: peterz@infradead.org (Peter Zijlstra)
Date: Tue, 26 Jan 2016 11:24:02 +0100
Subject: [v3,11/41] mips: reuse asm-generic/barrier.h
In-Reply-To: <20160114222046.GH3818@linux.vnet.ibm.com>
References: <569565DA.2010903@imgtec.com> <20160113104516.GE25458@arm.com>
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 <20160113204844.GV6357@twins.programming.kicks-ass.net>
 <5696BA6E.4070508@imgtec.com> <20160114120445.GB15828@arm.com>
 <56980145.5030901@imgtec.com>
 <20160114204827.GE3818@linux.vnet.ibm.com>
 <56981212.7050301@imgtec.com>
 <20160114222046.GH3818@linux.vnet.ibm.com>
Message-ID: <20160126102402.GE6357@twins.programming.kicks-ass.net>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

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 <paulmck@linux.vnet.ibm.com>
> 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?