Date: Thu, 8 Jun 2017 20:24:35 -0700
From: Krister Johansen <kjlx@templeofstupid.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Krister Johansen <kjlx@templeofstupid.com>,
        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, peterz@infradead.org, rostedt@goodmis.org,
        dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com,
        oleg@redhat.com, bobby.prani@gmail.com, stable@vger.kernel.org,
        gregkh@linuxfoundation.org
Subject: Re: [PATCH tip/core/rcu 45/88] rcu: Add memory barriers for NOCB
 leader wakeup
Message-ID: <20170609032435.GE2553@templeofstupid.com>
References: <20170525215934.GA11578@linux.vnet.ibm.com>
 <1495749601-21574-45-git-send-email-paulmck@linux.vnet.ibm.com>
 <20170608201148.GA2553@templeofstupid.com>
 <20170608205500.GC3721@linux.vnet.ibm.com>
 <20170608212814.GD2553@templeofstupid.com>
 <20170608234743.GE3721@linux.vnet.ibm.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20170608234743.GE3721@linux.vnet.ibm.com>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1872
Lines: 40

On Thu, Jun 08, 2017 at 04:47:43PM -0700, Paul E. McKenney wrote:
> On Thu, Jun 08, 2017 at 02:28:14PM -0700, Krister Johansen wrote:
> > On Thu, Jun 08, 2017 at 01:55:00PM -0700, Paul E. McKenney wrote:
> > > Interesting!  This is the first that I have heard that this was anything
> > > other than a theoretical bug.  To the comment in your second URL, it is
> > > wise to recall that a seismologist was in fact arrested for failing to
> > > predict an earthquake.  Later acquitted/pardoned/whatever, but arrested
> > > nonetheless.  ;-)
> > 
> > Point taken.  I do realize that we all make mistakes, and certainly I do
> > too.
> 
> Indeed!  Let's just say that the author of that email will have no
> trouble returning the favor, and sooner rather than later.  ;-)

No doubt he's polishing up an extra small extra tight pair of handcuffs
with my name on them.

> >       Perhaps I should have said that my survey of current callers of
> > swake_up() was enough to convince me that I didn't have an immediate
> > problem elsewhere, but that I'm not familiar enough with the code base
> > to make that statement with a lot of authority.  The concern being that if
> > the patch came from RT-linux where the barrier was present in
> > swake_up(), are there other places where swake_up() callers still assume
> > this is being handled on their behalf?
> > 
> > As part of this, I also pondered whether I should add a comment around
> > swake_up(), similar to what's already there for waitqueue_active.
> > I wasn't sure how subtle this is for other consumers, though.
> 
> In my case, I assume I need barriers for swake_up(), which is why I
> found this bug by inspection.  Still, I wouldn't mind a comment.
> Others might have other opinions.

Since you don't mind, I've prepared a small patch for those comments.  I'll
send that in a separate thread.

Thanks again,

-K