linux-kernel - Re: [PATCH tip/core/rcu 01/21] doc: READ_ONCE() now implies smp_barrier

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Message-Id: <20171204185215.GB7829@linux.vnet.ibm.com>
Date:   Mon, 4 Dec 2017 10:52:15 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     David Howells <dhowells@...hat.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com
Subject: Re: [PATCH tip/core/rcu 01/21] doc: READ_ONCE() now implies
 smp_barrier_depends()

On Mon, Dec 04, 2017 at 03:38:56PM +0000, David Howells wrote:
> Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > -	Q = READ_ONCE(P); smp_read_barrier_depends(); D = READ_ONCE(*Q);
> > +	Q = READ_ONCE(P); D = READ_ONCE(*Q);
> >  
> >       the CPU will issue the following memory operations:
> >  
> >  	Q = LOAD P, D = LOAD *Q
> 
> The CPU may now issue two barriers in addition to the loads, so should we show
> this?  E.g.:
> 
> 	Q = LOAD P, BARRIER, D = LOAD *Q, BARRIER

Good point!  How about as shown in the updated patch below?

							Thanx, Paul

------------------------------------------------------------------------

commit 40555946447a394889243e4393e312f65d847e1e
Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Date:   Mon Oct 9 09:15:21 2017 -0700

    doc: READ_ONCE() now implies smp_barrier_depends()
    
    This commit updates an example in memory-barriers.txt to account for
    the fact that READ_ONCE() now implies smp_barrier_depends().
    
    Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
    [ paulmck: Added MEMORY_BARRIER instructions from DEC Alpha from
      READ_ONCE(), per David Howells's feedback. ]

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 479ecec80593..13fd35b6a597 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -227,17 +227,20 @@ There are some minimal guarantees that may be expected of a CPU:
  (*) On any given CPU, dependent memory accesses will be issued in order, with
      respect to itself.  This means that for:
 
-	Q = READ_ONCE(P); smp_read_barrier_depends(); D = READ_ONCE(*Q);
+	Q = READ_ONCE(P); D = READ_ONCE(*Q);
 
      the CPU will issue the following memory operations:
 
 	Q = LOAD P, D = LOAD *Q
 
-     and always in that order.  On most systems, smp_read_barrier_depends()
-     does nothing, but it is required for DEC Alpha.  The READ_ONCE()
-     is required to prevent compiler mischief.  Please note that you
-     should normally use something like rcu_dereference() instead of
-     open-coding smp_read_barrier_depends().
+     and always in that order.  However, on DEC Alpha, READ_ONCE() also
+     emits a memory-barrier instruction, so that a DEC Alpha CPU will
+     instead issue the following memory operations:
+
+	Q = LOAD P, MEMORY_BARRIER, D = LOAD *Q, MEMORY_BARRIER
+
+     Whether on DEC Alpha or not, the READ_ONCE() also prevents compiler
+     mischief.
 
  (*) Overlapping loads and stores within a particular CPU will appear to be
      ordered within that CPU.  This means that for: