Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 14 May 2018 06:25:41 -0700
From:   "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Xiao Guangrong <guangrong.xiao@gmail.com>, rostedt@goodmis.org,
        Lai Jiangshan <jiangshanlai@gmail.com>, stefani@seibold.net,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: Is read barrier missed in kfifo?
Reply-To: paulmck@linux.vnet.ibm.com
References: <5382d3a8-08e0-5429-0cd3-36f9c69197e1@gmail.com>
 <20180511083242.GJ12217@hirez.programming.kicks-ass.net>
 <20180511162053.GY26088@linux.vnet.ibm.com>
 <20180514065755.GM12217@hirez.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20180514065755.GM12217@hirez.programming.kicks-ass.net>
User-Agent: Mutt/1.5.21 (2010-09-15)
Message-Id: <20180514132541.GS26088@linux.vnet.ibm.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, May 14, 2018 at 08:57:55AM +0200, Peter Zijlstra wrote:
> On Fri, May 11, 2018 at 09:20:53AM -0700, Paul E. McKenney wrote:
> > On Fri, May 11, 2018 at 10:32:42AM +0200, Peter Zijlstra wrote:
> > > On Fri, May 11, 2018 at 03:25:18PM +0800, Xiao Guangrong wrote:
> > > > 
> > > > Hi,
> > > > 
> > > > Currently, there is no read barrier between reading the index
> > > > (kfifo.in) and fetching the real data from the fifo.
> > > > 
> > > > I am afraid that will cause the vfifo is observed as not empty
> > > > however the data is not actually ready for read. Right?
> > > 
> > > That code is decidedly dodgy indeed. I can only see smp_wmb() but no
> > > matching barriers at all -- therefore the code is almost certainly as
> > > good as not having any barriers at all.
> > > 
> > > I would suggest you try and convert the code to smp_store_release() and
> > > smp_load_acquire() while you're at it.
> > 
> > Isn't this one of the places where we rely on control dependencies?
> 
> Then it bloody well should have a comment. But at least one side of the
> fifo needs a read barrier I think. We can rely on a ctrl-dep on the
> write side, where we read the head/tail values, compute space and then
> conditionally allow writes to happen.
> 
> But on the read side it's all reads and ctrl-dep doesn't help anything.

Agreed, for a control depdendency to help, there does need to be a
control dependency.  ;-)

							Thanx, Paul