Date:   Thu, 29 Jul 2021 11:05:17 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     rcu <rcu@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...com>, Ingo Molnar <mingo@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Josh Triplett <josh@...htriplett.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        rostedt <rostedt@...dmis.org>,
        David Howells <dhowells@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        fweisbec <fweisbec@...il.com>, Oleg Nesterov <oleg@...hat.com>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 rcu 04/18] rcu: Weaken ->dynticks accesses and updates

On Thu, Jul 29, 2021 at 01:41:41PM -0400, Mathieu Desnoyers wrote:
> ----- On Jul 29, 2021, at 11:57 AM, paulmck paulmck@...nel.org wrote:
> 
> > On Thu, Jul 29, 2021 at 10:41:18AM -0400, Mathieu Desnoyers wrote:
> >> ----- On Jul 28, 2021, at 4:28 PM, paulmck paulmck@...nel.org wrote:
> >> 
> >> > On Wed, Jul 28, 2021 at 04:03:02PM -0400, Mathieu Desnoyers wrote:
> >> >> ----- On Jul 28, 2021, at 3:45 PM, paulmck paulmck@...nel.org wrote:
> >> >> [...]
> >> >> > 
> >> >> > And how about like this?
> >> >> > 
> >> >> >						Thanx, Paul
> >> >> > 
> >> >> > ------------------------------------------------------------------------
> >> >> > 
> >> >> > commit cb8914dcc6443cca15ce48d937a93c0dfdb114d3
> >> >> > Author: Paul E. McKenney <paulmck@...nel.org>
> >> >> > Date:   Wed Jul 28 12:38:42 2021 -0700
> >> >> > 
> >> >> >    rcu: Move rcu_dynticks_eqs_online() to rcu_cpu_starting()
> >> >> >    
> >> >> >    The purpose of rcu_dynticks_eqs_online() is to adjust the ->dynticks
> >> >> >    counter of an incoming CPU if required.  It is currently is invoked
> >> >> 
> >> >> "is currently is" -> "is currently"
> >> > 
> >> > Good catch, fixed!
> >> > 
> >> >> >    from rcutree_prepare_cpu(), which runs before the incoming CPU is
> >> >> >    running, and thus on some other CPU.  This makes the per-CPU accesses in
> >> >> >    rcu_dynticks_eqs_online() iffy at best, and it all "works" only because
> >> >> >    the running CPU cannot possibly be in dyntick-idle mode, which means
> >> >> >    that rcu_dynticks_eqs_online() never has any effect.  One could argue
> >> >> >    that this means that rcu_dynticks_eqs_online() is unnecessary, however,
> >> >> >    removing it makes the CPU-online process vulnerable to slight changes
> >> >> >    in the CPU-offline process.
> >> >> 
> >> >> Why favor moving this from the prepare_cpu to the cpu_starting hotplug step,
> >> >> rather than using the target cpu's rdp from rcutree_prepare_cpu ? Maybe there
> >> >> was a good reason for having this very early in the prepare_cpu step ?
> >> > 
> >> > Some years back, there was a good reason. This reason was that
> >> > rcutree_prepare_cpu() marked the CPU as being online from an RCU
> >> > viewpoint.  But now rcu_cpu_starting() is the one that marks the CPU as
> >> > being online, so the ->dynticks check can be deferred to this function.
> >> > 
> >> >> Also, the commit message refers to this bug as having no effect because the
> >> >> running CPU cannot possibly be in dyntick-idle mode. I understand that calling
> >> >> this function was indeed effect-less, but then why is it OK for the CPU coming
> >> >> online to skip this call in the first place ? This commit message hints at
> >> >> "slight changes in the CPU-offline process" which could break it, but therer is
> >> >> no explanation of what makes this not an actual bug fix.
> >> > 
> >> > Because rcutorture would not have suffered in silence had this
> >> > situation ever arisen.
> >> 
> >> Testing can usually prove the presence of a bug, but it's rather tricky to prove
> >> the absence of bug.
> > 
> > In general, true enough.
> > 
> > But in this particular case, a WARN would have deterministically triggered
> > the very next time that this CPU found its way either to the idle loop
> > or to nohz_full usermode execution.
> > 
> >> > I have updated the commit log to answer these questions as shown
> >> > below.  Thoughts?
> >> 
> >> I'm still concerned about one scenario wrt moving rcu_dynticks_eqs_online()
> >> from rcutree_prepare_cpu to rcu_cpu_starting. What happens if an interrupt
> >> handler, or a NMI handler, nests early over the CPU-online startup code ?
> >> AFAIU, this interrupt handler could contain RCU read-side critical sections,
> >> but if the eqs state does not show the CPU as "online", I wonder whether it
> >> will work as expected.
> > 
> > Interrupts are still disabled at this point in the onlining process,
> > my _irqsave() locks notwithstanding.
> > 
> > You are right about NMI handlers, but there would be much more damage
> > from an early NMI handler than mere RCU issues.  And this can be handled
> > as described in the next paragraph.
> > 
> > Now, there are architectures (including x86) that need RCU readers
> > before notify_cpu_starting() time (which is where rcu_cpu_starting()
> > is invoked by default, and those architectures can (and do) simply
> > place a call to rcu_cpu_starting() at an earlier appropriate point in
> > the architecture-specific CPU-bringup code.  And this is in fact the
> > reason for the ->cpu_started check at the beginning of rcu_cpu_starting().
> > So an architecture using NMIs early in the CPU-bringup code should
> > invoke rcu_cpu_starting() before enabling NMIs.
> > 
> > So why not just be safe and mark the CPU online early in the process?
> > 
> > Because that could result in unbounded grace periods and strange
> > deadlocks.  These deadlocks were broken earlier by code that assumed that
> > a CPU could not possibly take more than one jiffy to come online, but that
> > assumption is clearly broken on todays systems, even the bare-metal ones.
> > 
> > In theory, I could change the raw_spin_lock_irqsave_rcu_node() to
> > raw_spin_lock_rcu_node(), rely on the lockdep_assert_irqs_disabled()
> > in the matching raw_spin_unlock_rcu_node(), and ditch the "flags"
> > local variable, but rcu_report_qs_rnp() needs that "flags" argument.
> > 
> > OK, I guess one approach is to get the "flags" value from local_save_flags()
> > and get rid of the _irqsave and _irq restore.  Assuming lockdep is
> > functional that early in CPU bringup.
> > 
> > But would that really be better than status quo?
> 
> I'm OK with your explanation about the fact that interrupts and NMIs scenarios
> are correctly handled, so moving this call from prepare_cpu to cpu_starting
> is fine with me.

I will add a "Link:" to this conversation.

May I also add your "Acked-by" or similar?

							Thanx, Paul

> Thanks,
> 
> Mathieu
> 
> > 
> >							Thanx, Paul
> > 
> >> Thanks,
> >> 
> >> Mathieu
> >> 
> >> > 
> >> >							Thanx, Paul
> >> > 
> >> > ------------------------------------------------------------------------
> >> > 
> >> > commit 516c8c4cc6fce62539f7e0182739812db4591c1d
> >> > Author: Paul E. McKenney <paulmck@...nel.org>
> >> > Date:   Wed Jul 28 12:38:42 2021 -0700
> >> > 
> >> >    rcu: Move rcu_dynticks_eqs_online() to rcu_cpu_starting()
> >> >    
> >> >    The purpose of rcu_dynticks_eqs_online() is to adjust the ->dynticks
> >> >    counter of an incoming CPU when required.  It is currently invoked
> >> >    from rcutree_prepare_cpu(), which runs before the incoming CPU is
> >> >    running, and thus on some other CPU.  This makes the per-CPU accesses in
> >> >    rcu_dynticks_eqs_online() iffy at best, and it all "works" only because
> >> >    the running CPU cannot possibly be in dyntick-idle mode, which means
> >> >    that rcu_dynticks_eqs_online() never has any effect.
> >> >    
> >> >    It is currently OK for rcu_dynticks_eqs_online() to have no effect, but
> >> >    only because the CPU-offline process just happens to leave ->dynticks in
> >> >    the correct state.  After all, if ->dynticks were in the wrong state on a
> >> >    just-onlined CPU, rcutorture would complain bitterly the next time that
> >> >    CPU went idle, at least in kernels built with CONFIG_RCU_EQS_DEBUG=y,
> >> >    for example, those built by rcutorture scenario TREE04.  One could
> >> >    argue that this means that rcu_dynticks_eqs_online() is unnecessary,
> >> >    however, removing it would make the CPU-online process vulnerable to
> >> >    slight changes in the CPU-offline process.
> >> >    
> >> >    One could also ask why it is safe to move the rcu_dynticks_eqs_online()
> >> >    call so late in the CPU-online process.  Indeed, there was a time when it
> >> >    would not have been safe, which does much to explain its current location.
> >> >    However, the marking of a CPU as online from an RCU perspective has long
> >> >    since moved from rcutree_prepare_cpu() to rcu_cpu_starting(), and all
> >> >    that is required is that ->dynticks be set correctly by the time that
> >> >    the CPU is marked as online from an RCU perspective.  After all, the RCU
> >> >    grace-period kthread does not check to see if offline CPUs are also idle.
> >> >    (In case you were curious, this is one reason why there is quiescent-state
> >> >    reporting as part of the offlining process.)
> >> >    
> >> >    This commit therefore moves the call to rcu_dynticks_eqs_online() from
> >> >    rcutree_prepare_cpu() to rcu_cpu_starting(), this latter being guaranteed
> >> >    to be running on the incoming CPU.  The call to this function must of
> >> >    course be placed before this rcu_cpu_starting() announces this CPU's
> >> >    presence to RCU.
> >> >    
> >> >    Reported-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> >> >    Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> >> > 
> >> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> >> > index 0172a5fd6d8de..aa00babdaf544 100644
> >> > --- a/kernel/rcu/tree.c
> >> > +++ b/kernel/rcu/tree.c
> >> > @@ -4129,7 +4129,6 @@ int rcutree_prepare_cpu(unsigned int cpu)
> >> > 	rdp->n_force_qs_snap = READ_ONCE(rcu_state.n_force_qs);
> >> > 	rdp->blimit = blimit;
> >> > 	rdp->dynticks_nesting = 1;	/* CPU not up, no tearing. */
> >> > -	rcu_dynticks_eqs_online();
> >> > 	raw_spin_unlock_rcu_node(rnp);		/* irqs remain disabled. */
> >> > 
> >> > 	/*
> >> > @@ -4249,6 +4248,7 @@ void rcu_cpu_starting(unsigned int cpu)
> >> > 	mask = rdp->grpmask;
> >> > 	WRITE_ONCE(rnp->ofl_seq, rnp->ofl_seq + 1);
> >> > 	WARN_ON_ONCE(!(rnp->ofl_seq & 0x1));
> >> > +	rcu_dynticks_eqs_online();
> >> > 	smp_mb(); // Pair with rcu_gp_cleanup()'s ->ofl_seq barrier().
> >> > 	raw_spin_lock_irqsave_rcu_node(rnp, flags);
> >> >  	WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext | mask);
> >> 
> >> --
> >> Mathieu Desnoyers
> >> EfficiOS Inc.
> > > http://www.efficios.com
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com

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