linux-kernel - Re: [PATCH tip/core/rcu 4/4] documentation: Update circular buffer for load-acquire/store-release

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Message-ID: <20131116154032.GC4138@linux.vnet.ibm.com>
Date:	Sat, 16 Nov 2013 07:40:32 -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,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, edumazet@...gle.com,
	darren@...art.com, fweisbec@...il.com, sbw@....edu
Subject: Re: [PATCH tip/core/rcu 4/4] documentation: Update circular buffer
 for load-acquire/store-release

On Sat, Nov 16, 2013 at 11:58:45AM +0000, David Howells wrote:
> Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > -		/* read index before reading contents at that index */
> 
> > -		smp_mb(); /* finish reading descriptor before incrementing tail */
> 
> I'd rather you didn't remove these comments (assuming they're correct) as
> they're pointing out the point of the example.

Ah, good point!  I have added them back just before the smp_load_acquire()
and smp_store_release(), as shown below.  Does that work?

							Thanx, Paul

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

diff --git a/Documentation/circular-buffers.txt b/Documentation/circular-buffers.txt
index 15e54ff50018..88951b179262 100644
--- a/Documentation/circular-buffers.txt
+++ b/Documentation/circular-buffers.txt
@@ -200,6 +200,7 @@ The consumer will look something like this:
 
 	spin_lock(&consumer_lock);
 
+	/* Read index before reading contents at that index. */
 	unsigned long head = smp_load_acquire(buffer->head);
 	unsigned long tail = buffer->tail;
 
@@ -210,6 +211,7 @@ The consumer will look something like this:
 
 		consume_item(item);
 
+		/* Finish reading descriptor before incrementing tail. */
 		smp_store_release(buffer->tail,
 				  (tail + 1) & (buffer->size - 1));
 	}

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