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Date:   Sat, 30 Sep 2023 09:30:32 +0200
From:   Frederic Weisbecker <frederic@kernel.org>
To:     Neeraj upadhyay <neeraj.iitr10@gmail.com>
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        zhuangel570 <zhuangel570@gmail.com>, paulmck@kernel.org,
        rcu@vger.kernel.org, josh@joshtriplett.org, rostedt@goodmis.org,
        mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com,
        like.xu.linux@gmail.com, linussli@tencent.com, foxywang@tencent.com
Subject: Re: SRCU: kworker hung in synchronize_srcu
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On Sat, Sep 30, 2023 at 08:15:06AM +0530, Neeraj upadhyay wrote:
> On Sat, Sep 30, 2023 at 4:15 AM Frederic Weisbecker <frederic@kernel.org> wrote:
> >
> > Le Thu, Sep 28, 2023 at 05:39:17PM -0400, Joel Fernandes a écrit :
> > > If srcu_invoke_callbacks() was really called for the rdp, I would have
> > > expected rcu_segcblist_advance() to advance all those pending
> > > callbacks to 304.
> > >
> > > I posit that probably srcu_invoke_callbacks() is not even being called
> > > in the first place, otherwise why is the DONE segment count still 0?
> > > DONE should not be 0, otherwise callbacks will not run. The segcblist
> > > of the rdp seems out of date perhaps due to rcu_invoke_callbacks() not
> > > being called due to some other bug (or a workqueue/timer issue causing
> > > the srcu_invoke_callbacks() to not run?).
> >
> > Speaking of, I'm looking at srcu_gp_end() and the comment after releasing
> > srcu_gp_mutex says:
> >
> >               "/* A new grace period can start at this point.  But only one. */"
> >
> > But what ensures that?
> >
> 
> I think ->srcu_cb_mutex ensures that.
> 
> /* Prevent more than one additional grace period. */
> mutex_lock(&sup->srcu_cb_mutex);

Yes but right ater it's relaxed (before callbacks are schedule for execution)?