linux-kernel - [PATCH v2 tip/core/rcu 13/18] rcu: Make rcu_cpu

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Date:   Tue, 17 Jan 2017 18:53:52 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
        bobby.prani@...il.com,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH v2 tip/core/rcu 13/18] rcu: Make rcu_cpu_starting() use its "cpu" argument

The rcu_cpu_starting() function uses this_cpu_ptr() to locate the
incoming CPU's rcu_data structure.  This works for the boot CPU and for
all CPUs onlined after rcu_init() executes (during very early boot).
Currently, this is the full set of CPUs, so all is well.  But if
anyone ever parallelizes boot before rcu_init() time, it will fail.
This commit therefore substitutes the rcu_cpu_starting() function's
this_cpu_pointer() for per_cpu_ptr(), future-proofing the code and
(arguably) improving readability.

This commit inadvertently fixes a latent bug: If there ever had been
more than just the boot CPU online at rcu_init() time, the old code
would not initialize the non-boot CPUs, but rather would repeatedly
initialize the boot CPU.

Reported-by: Boqun Feng <boqun.feng@...il.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@...htriplett.org>
---
 kernel/rcu/tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index be2301238a23..a4b4762442bb 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3873,7 +3873,7 @@ void rcu_cpu_starting(unsigned int cpu)
 	struct rcu_state *rsp;

 	for_each_rcu_flavor(rsp) {
-		rdp = this_cpu_ptr(rsp->rda);
+		rdp = per_cpu_ptr(rsp->rda, cpu);
 		rnp = rdp->mynode;
 		mask = rdp->grpmask;
 		raw_spin_lock_irqsave_rcu_node(rnp, flags);
-- 
2.5.2