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

["Paul E. McKenney" <paulmck@kernel.org>]

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.

I have updated the commit log to answer these questions as shown
below.  Thoughts?

							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);