linux-kernel - RE: [PATCH] rcu-tasks: Directly invoke rcuwait_wake_up() in call_rcu_tasks

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Date:   Fri, 24 Feb 2023 03:04:58 +0000
From:   "Zhang, Qiang1" <qiang1.zhang@...el.com>
To:     Joel Fernandes <joel@...lfernandes.org>
CC:     "Zhuo, Qiuxu" <qiuxu.zhuo@...el.com>,
        "paulmck@...nel.org" <paulmck@...nel.org>,
        "frederic@...nel.org" <frederic@...nel.org>,
        "quic_neeraju@...cinc.com" <quic_neeraju@...cinc.com>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] rcu-tasks: Directly invoke rcuwait_wake_up() in
 call_rcu_tasks_generic()

On Thu, Feb 23, 2023 at 9:35 PM Joel Fernandes <joel@...lfernandes.org> wrote:
>
> On Thu, Feb 23, 2023 at 9:25 PM Joel Fernandes <joel@...lfernandes.org> wrote:
> >
> > On Fri, Feb 24, 2023 at 12:36:05AM +0000, Zhang, Qiang1 wrote:
> > > On Thu, Feb 23, 2023 at 08:43:05AM +0000, Zhang, Qiang1 wrote:
> > > > > From: Zqiang <qiang1.zhang@...el.com>
> > > > > Sent: Thursday, February 23, 2023 2:30 PM
> > > > > To: paulmck@...nel.org; frederic@...nel.org; quic_neeraju@...cinc.com;
> > > > > joel@...lfernandes.org
> > > > > Cc: rcu@...r.kernel.org; linux-kernel@...r.kernel.org
> > > > > Subject: [PATCH] rcu-tasks: Directly invoke rcuwait_wake_up() in
> > > > > call_rcu_tasks_generic()
> > > > >
> > > > > According to commit '3063b33a347c ("Avoid raw-spinlocked wakeups from
> > > > > call_rcu_tasks_generic()")', the grace-period kthread is delayed to wakeup
> > > > > using irq_work_queue() is because if the caller of
> > > > > call_rcu_tasks_generic() holds a raw spinlock, when the kernel is built with
> > > > > CONFIG_PROVE_RAW_LOCK_NESTING=y, due to a spinlock will be hold in
> > > > > wake_up(), so the lockdep splats will happen. but now using
> > > > > rcuwait_wake_up() to wakeup grace-period kthread instead of wake_up(), in
> > > > > rcuwait_wake_up() no spinlock will be acquired, so this commit remove using
> > > > >
> > > > >There are still spinlock-acquisition and spinlock-release invocations within the call path from rcuwait_wake_up().
> > > > >
> > > > >rcuwait_wake_up() -> wake_up_process() -> try_to_wake_up(), then:
> > > > >
> > > > >    raw_spin_lock_irqsave()
> > > > >    ...
> > > > >    raw_spin_unlock_irqrestore
> > > >
> > > > Yes, but this is raw_spinlock acquisition and release(note: spinlock will convert to
> > > > sleepable lock in Preempt-RT kernel, but raw spinlock is not change).
> > > >
> > > > acquire raw_spinlock -> acquire spinlock  will trigger lockdep warning.
> > > >
> > > >Is this really safe in the long run though? I seem to remember there are
> > > >weird locking dependencies if RCU is used from within the scheduler [1].
> > > >
> > >
> > >
> > > I have  been running rcutorture with rcutorture.type = tasks-tracing,
> > > so far no problems have been found.
> > >
> > >
> > > >I prefer to keep it as irq_work_queue() unless you are seeing some benefit.
> > > >Generally, there has to be a 'win' or other justification for adding more
> > > >risk.
> > > >
> > > >thanks,
> > > >
> > > >- Joel
> > > >[1] http://www.joelfernandes.org/rcu/scheduler/locking/2019/09/02/rcu-schedlocks.html
> > >
> > >
> > > The problem in this link,  in an earlier RCU version, rcu_read_unlock_special()
> > > Invoke wakeup and enter scheduler can lead to deadlock, but my modification is for
> > > call_rcu_tasks_generic(), even if there is a lock dependency problem, we should pay
> > > more attention to rcu_read_unlock_trace_special()
> >
> > Consider ABBA deadlocks as well, not just self-deadlocks (which IIRC is what
> > the straight-RCU rcu_read_unlock() issues were about).
> >
> > What prevents the following scenario?
> >
> > In the scheduler you have code like this:
> >                 rq = task_rq_lock(p, &rf);
> >                 trace_sched_wait_task(p);
> >
> > Someone can hook up a BPF program to that tracepoint that then calls
> > rcu_read_unlock_trace() -> rcu_read_unlock_trace_special(). All of
> > this while holding the rq and pi scheduler locks.
> >
> > That's A (rq lock) -> B (rtpcp lock).

In rcu_read_unlock_trace_special(), the premise of acquiring the rtpcp lock is that
before that, we have task switch in the rcu_read_lock_trace/unlock_trace critical section.
but after we already hold the rq lock, no task switch is generated in the
rcu_read_lock_trace/unlock_trace  critical section.

Please correct me if my understanding is wrong.

Thanks
Zqiang

> >
> > In another path, your change adds the following dependency due to doing
> > wakeup under the rtpcp lock.
> >
> > That's call_rcu_tasks_generic() -> B (rtpcp lock) -> A (rq lock).
>
> I would like to correct this last statement. That cannot happen but
> the concern I guess is, can the following happen due to the change?
>
> call_rcu_tasks_generic() -> B (some BPF lock) -> A (rq lock)
>
>
>Aaargh, one more correction:
>B (some BPF lock) -> call_rcu_tasks_generic() ->  -> A (rq lock)
>
> ;-)
>
>-Joel