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Date:   Fri, 28 Jun 2019 18:28:06 +0900
From:   Byungchul Park <byungchul.park@lge.com>
To:     "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc:     Scott Wood <swood@redhat.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        rcu <rcu@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>, kernel-team@lge.com
Subject: Re: [RFC] Deadlock via recursive wakeup via RCU with threadirqs
Message-ID: <20190628092806.GB11339@X58A-UD3R>
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On Fri, Jun 28, 2019 at 06:10:42PM +0900, Byungchul Park wrote:
> On Fri, Jun 28, 2019 at 04:43:50PM +0900, Byungchul Park wrote:
> > On Fri, Jun 28, 2019 at 04:31:38PM +0900, Byungchul Park wrote:
> > > On Thu, Jun 27, 2019 at 01:36:12PM -0700, Paul E. McKenney wrote:
> > > > On Thu, Jun 27, 2019 at 03:17:27PM -0500, Scott Wood wrote:
> > > > > On Thu, 2019-06-27 at 11:41 -0700, Paul E. McKenney wrote:
> > > > > > On Thu, Jun 27, 2019 at 02:16:38PM -0400, Joel Fernandes wrote:
> > > > > > > 
> > > > > > > I think the fix should be to prevent the wake-up not based on whether we
> > > > > > > are
> > > > > > > in hard/soft-interrupt mode but that we are doing the rcu_read_unlock()
> > > > > > > from
> > > > > > > a scheduler path (if we can detect that)
> > > > > > 
> > > > > > Or just don't do the wakeup at all, if it comes to that.  I don't know
> > > > > > of any way to determine whether rcu_read_unlock() is being called from
> > > > > > the scheduler, but it has been some time since I asked Peter Zijlstra
> > > > > > about that.
> > > > > > 
> > > > > > Of course, unconditionally refusing to do the wakeup might not be happy
> > > > > > thing for NO_HZ_FULL kernels that don't implement IRQ work.
> > > > > 
> > > > > Couldn't smp_send_reschedule() be used instead?
> > > > 
> > > > Good point.  If current -rcu doesn't fix things for Sebastian's case,
> > > > that would be well worth looking at.  But there must be some reason
> > > > why Peter Zijlstra didn't suggest it when he instead suggested using
> > > > the IRQ work approach.
> > > > 
> > > > Peter, thoughts?
> > > 
> > 
> > +cc kernel-team@lge.com
> > (I'm sorry for more noise on the thread.)
> > 
> > > Hello,
> > > 
> > > Isn't the following scenario possible?
> > > 
> > > The original code
> > > -----------------
> > > rcu_read_lock();
> > > ...
> > > /* Experdite */
> > > WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
> > > ...
> > > __rcu_read_unlock();
> > > 	if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
> > > 		rcu_read_unlock_special(t);
> > > 			WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
> > > 			rcu_preempt_deferred_qs_irqrestore(t, flags);
> > > 		barrier();  /* ->rcu_read_unlock_special load before assign */
> > > 		t->rcu_read_lock_nesting = 0;
> > > 
> > > The reordered code by machine
> > > -----------------------------
> > > rcu_read_lock();
> > > ...
> > > /* Experdite */
> > > WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
> > > ...
> > > __rcu_read_unlock();
> > > 	if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
> > > 		rcu_read_unlock_special(t);
> > > 		t->rcu_read_lock_nesting = 0; <--- LOOK AT THIS!!!
> > > 			WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
> > > 			rcu_preempt_deferred_qs_irqrestore(t, flags);
> > > 		barrier();  /* ->rcu_read_unlock_special load before assign */
> > > 
> > > An interrupt happens
> > > --------------------
> > > rcu_read_lock();
> > > ...
> > > /* Experdite */
> > > WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
> > > ...
> > > __rcu_read_unlock();
> > > 	if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
> > > 		rcu_read_unlock_special(t);
> > > 		t->rcu_read_lock_nesting = 0; <--- LOOK AT THIS!!!
> > > <--- Handle an (any) irq
> > > 	rcu_read_lock();
> > > 	/* This call should be skipped */
> > > 	rcu_read_unlock_special(t);
> > > 			WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
> > > 			rcu_preempt_deferred_qs_irqrestore(t, flags);
> > > 		barrier();  /* ->rcu_read_unlock_special load before assign */
> 
> I was confused it was a LOAD access. The example should be changed a bit.
> 
> 
> 
> The original code
> -----------------
> rcu_read_lock();
> ...
> /* Experdite */
> WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
> ...
> __rcu_read_unlock();
> 	if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
> 		rcu_read_unlock_special(t);
> 			WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
> 			rcu_preempt_deferred_qs_irqrestore(t, flags);
> 		barrier();  /* ->rcu_read_unlock_special load before assign */
> 		t->rcu_read_lock_nesting = 0;
> 
> The reordered code by machine
> -----------------------------
> rcu_read_lock();
> ...
> /* Experdite */
> WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
> ...
> __rcu_read_unlock();
> 	if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
> 		rcu_read_unlock_special(t);
> 			rcu_preempt_deferred_qs_irqrestore(t, flags);
> 		barrier();  /* ->rcu_read_unlock_special load before assign */
> 		t->rcu_read_lock_nesting = 0;
> 			WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
> 
> An interrupt happens
> --------------------
> rcu_read_lock();
> ...
> /* Experdite */
> WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
> ...
> __rcu_read_unlock();
> 	if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
> 		rcu_read_unlock_special(t);
> 			rcu_preempt_deferred_qs_irqrestore(t, flags);
> 		barrier();  /* ->rcu_read_unlock_special load before assign */
> 		t->rcu_read_lock_nesting = 0;
> <--- Handle an (any) irq
> rcu_read_lock();
> /* This call should be skipped */
> rcu_read_unlock_special(t);
> 			WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
> 
> 
> 
> Now that I re-made the example, I'm afraid it'd be no problem because
> anyway it'd be within a cpu so it can see inside of the store-buffer of
> the cpu.
> 
> I'm sorry. Please ignore my suggestion here.

Even though the example is wrong but I think you can get confused with
about what I was trying to tell. It was about (1) LOAD in advance and
(2) reordering within a store buffer within a cpu, but not about
reordering instructions - I wrote the example as if it's about the
latter though.

Sorry for noise again.

Thanks,
Byungchul