linux-kernel - Re: [PATCH V2 2/7] rcu: cleanup rcu_preempt_deferred

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Date:   Mon, 4 Nov 2019 06:55:39 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Lai Jiangshan <laijs@...ux.alibaba.com>
Cc:     Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org
Subject: Re: [PATCH V2 2/7] rcu: cleanup rcu_preempt_deferred_qs()

On Sun, Nov 03, 2019 at 01:01:21PM +0800, Lai Jiangshan wrote:
> 
> 
> On 2019/11/3 10:01 上午, Boqun Feng wrote:
> > Hi Jiangshan,
> > 
> > 
> > I haven't checked the correctness of this patch carefully, but..
> > 
> > 
> > On Sat, Nov 02, 2019 at 12:45:54PM +0000, Lai Jiangshan wrote:
> > > Don't need to set ->rcu_read_lock_nesting negative, irq-protected
> > > rcu_preempt_deferred_qs_irqrestore() doesn't expect
> > > ->rcu_read_lock_nesting to be negative to work, it even
> > > doesn't access to ->rcu_read_lock_nesting any more.
> > 
> > rcu_preempt_deferred_qs_irqrestore() will report RCU qs, and may
> > eventually call swake_up() or its friends to wake up, say, the gp
> > kthread, and the wake up functions could go into the scheduler code
> > path which might have RCU read-side critical section in it, IOW,
> > accessing ->rcu_read_lock_nesting.
> 
> Sure, thank you for pointing it out.
> 
> I should rewrite the changelog in next round. Like this:
> 
> rcu: cleanup rcu_preempt_deferred_qs()
> 
> IRQ-protected rcu_preempt_deferred_qs_irqrestore() itself doesn't
> expect ->rcu_read_lock_nesting to be negative to work.
> 
> There might be RCU read-side critical section in it (from wakeup()
> or so), 1711d15bf5ef(rcu: Clear ->rcu_read_unlock_special only once)
> will ensure that ->rcu_read_unlock_special is zero and these RCU
> read-side critical sections will not call rcu_read_unlock_special().
> 
> Thanks
> Lai
> 
> ===
> PS: Were 1711d15bf5ef(rcu: Clear ->rcu_read_unlock_special only once)
> not applied earlier, it will be protected by previous patch (patch1)
> in this series
> "rcu: use preempt_count to test whether scheduler locks is held"
> when rcu_read_unlock_special() is called.

This one in -rcu, you mean?

5c5d9065e4eb ("rcu: Clear ->rcu_read_unlock_special only once")

Some adjustment was needed due to my not applying the earlier patches
that assumed nested interrupts.  Please let me know if further adjustments
are needed.

							Thanx, Paul

> > Again, haven't checked closely, but this argument in the commit log
> > seems untrue.
> > 
> > Regards,
> > Boqun
> > 
> > > 
> > > It is true that NMI over rcu_preempt_deferred_qs_irqrestore()
> > > may access to ->rcu_read_lock_nesting, but it is still safe
> > > since rcu_read_unlock_special() can protect itself from NMI.
> > > 
> > > Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
> > > ---
> > >   kernel/rcu/tree_plugin.h | 5 -----
> > >   1 file changed, 5 deletions(-)
> > > 
> > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > > index aba5896d67e3..2fab8be2061f 100644
> > > --- a/kernel/rcu/tree_plugin.h
> > > +++ b/kernel/rcu/tree_plugin.h
> > > @@ -552,16 +552,11 @@ static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
> > >   static void rcu_preempt_deferred_qs(struct task_struct *t)
> > >   {
> > >   	unsigned long flags;
> > > -	bool couldrecurse = t->rcu_read_lock_nesting >= 0;
> > >   	if (!rcu_preempt_need_deferred_qs(t))
> > >   		return;
> > > -	if (couldrecurse)
> > > -		t->rcu_read_lock_nesting -= RCU_NEST_BIAS;
> > >   	local_irq_save(flags);
> > >   	rcu_preempt_deferred_qs_irqrestore(t, flags);
> > > -	if (couldrecurse)
> > > -		t->rcu_read_lock_nesting += RCU_NEST_BIAS;
> > >   }
> > >   /*
> > > -- 
> > > 2.20.1
> > >