From: "Paul E. McKenney" <paulmck@kernel.org>
To: Lai Jiangshan <laijs@linux.alibaba.com>
Cc: Boqun Feng <boqun.feng@gmail.com>,
linux-kernel@vger.kernel.org,
Josh Triplett <josh@joshtriplett.org>,
Steven Rostedt <rostedt@goodmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Joel Fernandes <joel@joelfernandes.org>,
rcu@vger.kernel.org
Subject: Re: [PATCH V2 2/7] rcu: cleanup rcu_preempt_deferred_qs()
Date: Mon, 4 Nov 2019 06:55:39 -0800 [thread overview]
Message-ID: <20191104145539.GY20975@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <7489f817-adaf-275b-b19d-18ad248b071f@linux.alibaba.com>
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@linux.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
> > >
next prev parent reply other threads:[~2019-11-04 14:55 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-02 12:45 [PATCH V2 0/7] rcu: introduce percpu rcu_preempt_depth Lai Jiangshan
2019-11-02 12:45 ` [PATCH V2 1/7] rcu: use preempt_count to test whether scheduler locks is held Lai Jiangshan
2019-11-15 16:53 ` Paul E. McKenney
2020-02-19 3:31 ` Joel Fernandes
2020-02-19 3:59 ` Paul E. McKenney
2019-11-02 12:45 ` [PATCH V2 2/7] rcu: cleanup rcu_preempt_deferred_qs() Lai Jiangshan
2019-11-03 2:01 ` Boqun Feng
2019-11-03 5:01 ` Lai Jiangshan
2019-11-04 14:55 ` Paul E. McKenney [this message]
2019-11-05 2:09 ` Lai Jiangshan
2019-11-05 7:19 ` Paul E. McKenney
2019-11-11 14:32 ` Paul E. McKenney
2019-11-12 1:28 ` Lai Jiangshan
2020-02-17 23:23 ` Paul E. McKenney
2020-02-18 14:41 ` Steven Rostedt
2020-02-18 16:43 ` Paul E. McKenney
2019-11-15 16:55 ` Paul E. McKenney
2019-11-02 12:45 ` [PATCH V2 3/7] rcu: remove useless special.b.deferred_qs Lai Jiangshan
2020-02-17 23:23 ` Paul E. McKenney
2019-11-02 12:45 ` [PATCH V2 4/7] rcu: don't use negative ->rcu_read_lock_nesting Lai Jiangshan
2020-02-17 23:26 ` Paul E. McKenney
2019-11-02 12:45 ` [PATCH V2 5/7] rcu: wrap usages of rcu_read_lock_nesting Lai Jiangshan
2019-11-15 22:25 ` Paul E. McKenney
2019-11-02 12:45 ` [PATCH V2 6/7] rcu: clear the special.b.need_qs in rcu_note_context_switch() Lai Jiangshan
2019-11-16 15:46 ` Paul E. McKenney
2019-11-02 12:45 ` [PATCH V2 7/7] x86,rcu: use percpu rcu_preempt_depth Lai Jiangshan
2019-11-02 16:30 ` Borislav Petkov
2019-11-03 4:33 ` Lai Jiangshan
2019-11-04 9:25 ` Sebastian Andrzej Siewior
2019-11-04 11:41 ` Lai Jiangshan
2019-11-04 12:09 ` Sebastian Andrzej Siewior
2019-11-16 15:48 ` Paul E. McKenney
2019-11-18 2:02 ` Lai Jiangshan
2019-11-18 14:59 ` Paul E. McKenney
2019-11-19 1:59 ` Lai Jiangshan
2019-11-19 21:14 ` Paul E. McKenney
2019-11-20 2:47 ` Lai Jiangshan
2019-11-21 4:02 ` Paul E. McKenney
2019-11-02 15:05 ` [PATCH V2 0/7] rcu: introduce " Paul E. McKenney
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