Re: [PATCH v2 rcu 04/18] rcu: Weaken ->dynticks accesses and updates

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Jul 28, 2021, at 4:28 PM, paulmck paulmck@kernel.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@kernel.org wrote:
>> [...]
>> > 
>> > And how about like this?
>> > 
>> >						Thanx, Paul
>> > 
>> > ------------------------------------------------------------------------
>> > 
>> > commit cb8914dcc6443cca15ce48d937a93c0dfdb114d3
>> > Author: Paul E. McKenney <paulmck@kernel.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.

> 
> 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.

Thanks,

Mathieu

> 
>							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit 516c8c4cc6fce62539f7e0182739812db4591c1d
> Author: Paul E. McKenney <paulmck@kernel.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@efficios.com>
>    Signed-off-by: Paul E. McKenney <paulmck@kernel.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