linux-kernel - Re: [PATCH tip/core/rcu 45/88] rcu: Add memory barriers for NOCB leader wakeup

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Date:   Thu, 8 Jun 2017 20:24:35 -0700
From:   Krister Johansen <kjlx@...pleofstupid.com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Krister Johansen <kjlx@...pleofstupid.com>,
        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, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com,
        stable@...r.kernel.org, gregkh@...uxfoundation.org
Subject: Re: [PATCH tip/core/rcu 45/88] rcu: Add memory barriers for NOCB
 leader wakeup

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