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From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Tue, 1 Dec 2020 17:26:32 -0500
Message-ID: <CAEXW_YSHVzzm=fLtamB=K5tjiwUNrYK76MxqfW2w0MJKO5LsTw@mail.gmail.com>
Subject: Re: [PATCH v2] rcu/segcblist: Add debug checks for segment lengths
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        rcu <rcu@vger.kernel.org>, Steven Rostedt <rostedt@goodmis.org>
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On Thu, Nov 19, 2020 at 3:42 PM Joel Fernandes <joel@joelfernandes.org> wrote:
>
> On Thu, Nov 19, 2020 at 12:16:15PM -0800, Paul E. McKenney wrote:
> > On Thu, Nov 19, 2020 at 02:44:35PM -0500, Joel Fernandes wrote:
> > > On Thu, Nov 19, 2020 at 2:22 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> > > > > > > > On Wed, Nov 18, 2020 at 11:15:41AM -0500, Joel Fernandes (Google) wrote:
> > > > > > > > > After rcu_do_batch(), add a check for whether the seglen counts went to
> > > > > > > > > zero if the list was indeed empty.
> > > > > > > > >
> > > > > > > > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > > > > > > >
> > > > > > > > Queued for testing and further review, thank you!
> > > > > > >
> > > > > > > FYI, the second of the two checks triggered in all four one-hour runs of
> > > > > > > TREE01, all four one-hour runs of TREE04, and one of the four one-hour
> > > > > > > runs of TREE07.  This one:
> > > > > > >
> > > > > > > WARN_ON_ONCE(count != 0 && rcu_segcblist_n_segment_cbs(&rdp->cblist) == 0);
> > > > > > >
> > > > > > > That is, there are callbacks in the list, but the sum of the segment
> > > > > > > counts is nevertheless zero.  The ->nocb_lock is held.
> > > > > > >
> > > > > > > Thoughts?
> > > > > >
> > > > > > FWIW, TREE01 reproduces it very quickly compared to the other two
> > > > > > scenarios, on all four run, within five minutes.
> > > > >
> > > > > So far for TREE01, I traced it down to an rcu_barrier happening so it could
> > > > > be related to some interaction with rcu_barrier() (Just a guess).
> > > >
> > > > Well, rcu_barrier() and srcu_barrier() are the only users of
> > > > rcu_segcblist_entrain(), if that helps.  Your modification to that
> > > > function looks plausible to me, but the system's opinion always overrules
> > > > mine.  ;-)
> > >
> > > Right. Does anything the bypass code standout? That happens during
> > > rcu_barrier() as well, and it messes with the lengths.
> >
> > In theory, rcu_barrier_func() flushes the bypass before doing the
> > entrain, and does the rcu_segcblist_entrain() afterwards.
> >
> > Ah, and that is the issue.  If ->cblist is empty and ->nocb_bypass
> > is not, then ->cblist length will be nonzero, and none of the
> > segments will be nonzero.
> >
> > So you need something like this for that second WARN, correct?
> >
> >       WARN_ON_ONCE(!rcu_segcblist_empty(&rdp->cblist) &&
> >                    rcu_segcblist_n_segment_cbs(&rdp->cblist) == 0);

Just started to look into it again. If the &rdp->cblist is empty, that
means the bypass list could not have been used (Since per comments on
rcu_nocb_try_bypass() , the bypass list is in use only when the cblist
is non-empty). So the cblist was non empty, then the segment counts
should not sum to 0.  So I don't think that explains it. Anyway, I
will try the new version of your warning in case there is something
about bypass lists that I'm missing.