Date: Tue, 5 Dec 2017 14:02:10 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org,
        mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com,
        akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
        josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org,
        dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com,
        oleg@redhat.com, Jason Wang <jasowang@redhat.com>, kvm@vger.kernel.org,
        virtualization@lists.linux-foundation.org, netdev@vger.kernel.org
Subject: Re: [PATCH tip/core/rcu 21/21] drivers/vhost: Remove now-redundant
 read_barrier_depends()
Reply-To: paulmck@linux.vnet.ibm.com
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On Tue, Dec 05, 2017 at 11:43:41PM +0200, Michael S. Tsirkin wrote:
> On Tue, Dec 05, 2017 at 01:36:44PM -0800, Paul E. McKenney wrote:
> > On Tue, Dec 05, 2017 at 11:24:49PM +0200, Michael S. Tsirkin wrote:
> > > On Tue, Dec 05, 2017 at 12:08:01PM -0800, Paul E. McKenney wrote:
> > > > On Tue, Dec 05, 2017 at 09:51:48PM +0200, Michael S. Tsirkin wrote:
> > > > > On Tue, Dec 05, 2017 at 11:33:39AM -0800, Paul E. McKenney wrote:
> > > > > > On Tue, Dec 05, 2017 at 09:24:21PM +0200, Michael S. Tsirkin wrote:
> > > > 
> > > > [ . . . ]
> > > > 
> > > > > > > and this barrier is no longer paired with anything until
> > > > > > > you realize there's a dependency barrier within READ_ONCE.
> > > > > > > 
> > > > > > > Barrier pairing was a useful tool to check code validity,
> > > > > > > maybe there are other, better tools now.
> > > > > > 
> > > > > > There are quite a few people who say that smp_store_release() is
> > > > > > easier for the tools to analyze than is smp_wmb().  My experience with
> > > > > > smp_read_barrier_depends() and rcu_dereference() leads me to believe
> > > > > > that they are correct.
> > > > > 
> > > > > OK, but smp_store_release is still not paired with anything since we
> > > > > rely on READ_ONCE to include the implicit dpendendency barrier.
> > > > 
> > > > Why wouldn't you consider the smp_store_release() to be paired with
> > > > the new improved READ_ONCE()?
> > > 
> > > READ_ONCE is really all over the place (some code literally replaced all
> > > memory accesses with READ/WRITE ONCE).
> > > 
> > > And I also prefer smp_wmb as it seems to be cheaper on ARM.
> > > 
> > > Would an API like WRITE_POINTER()/smp_store_pointer make sense,
> > > and READ_POINTER for symmetry?
> > 
> > What we do in some code is to comment the pairings, allowing the other
> > side of the pairing to be easily located.  Would that work for you?
> 
> Yes, that's exactly what I did for now.

Very good, thank you!

							Thanx, Paul