[tip: core/rcu] rcu: Add READ_ONCE() to rcu_do_batch() access to rcu_divisor

[tip-bot2 for Paul E. McKenney]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     b5374b2df0ac1c78895b8eb8d9582a7bdc67257d
Gitweb:        https://git.kernel.org/tip/b5374b2df0ac1c78895b8eb8d9582a7bdc67257d
Author:        Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate:    Tue, 23 Jun 2020 17:09:27 -07:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Mon, 24 Aug 2020 18:36:06 -07:00

rcu: Add READ_ONCE() to rcu_do_batch() access to rcu_divisor

Given that sysfs can change the value of rcu_divisor at any time, this
commit adds a READ_ONCE to the sole access to that variable.  While in
the area, this commit also adds bounds checking, clamping the value to
a shift that makes sense for a signed long.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 kernel/rcu/tree.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index acc926f..1dca14c 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2362,6 +2362,7 @@ int rcutree_dead_cpu(unsigned int cpu)
  */
 static void rcu_do_batch(struct rcu_data *rdp)
 {
+	int div;
 	unsigned long flags;
 	const bool offloaded = IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
 			       rcu_segcblist_is_offloaded(&rdp->cblist);
@@ -2390,7 +2391,9 @@ static void rcu_do_batch(struct rcu_data *rdp)
 	rcu_nocb_lock(rdp);
 	WARN_ON_ONCE(cpu_is_offline(smp_processor_id()));
 	pending = rcu_segcblist_n_cbs(&rdp->cblist);
-	bl = max(rdp->blimit, pending >> rcu_divisor);
+	div = READ_ONCE(rcu_divisor);
+	div = div < 0 ? 7 : div > sizeof(long) * 8 - 2 ? sizeof(long) * 8 - 2 : div;
+	bl = max(rdp->blimit, pending >> div);
 	if (unlikely(bl > 100))
 		tlimit = local_clock() + rcu_resched_ns;
 	trace_rcu_batch_start(rcu_state.name,