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

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

----- 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"

>    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 ?

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.

Thanks,

Mathieu

>    
>    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