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>

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);