From: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
rcu_read_unlock_strict() can be called with preemption enabled
which can make for an unstable rdp and a racy norm value.
Fix this by dropping the preempt-count in __rcu_read_unlock()
after the call to rcu_read_unlock_strict(), adjusting the
preempt-count check appropriately.
Suggested-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
include/linux/rcupdate.h | 2 +-
kernel/rcu/tree_plugin.h | 11 ++++++++++-
2 files changed, 11 insertions(+), 2 deletions(-)
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index c6c4aee47df2..9b05db8ff061 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -95,9 +95,9 @@ static inline void __rcu_read_lock(void)
static inline void __rcu_read_unlock(void)
{
- preempt_enable();
if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))
rcu_read_unlock_strict();
+ preempt_enable();
}
static inline int rcu_preempt_depth(void)
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index bb7ca6eb9ef0..b025e95ede05 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -833,8 +833,17 @@ void rcu_read_unlock_strict(void)
{
struct rcu_data *rdp;
- if (irqs_disabled() || preempt_count() || !rcu_state.gp_kthread)
+ if (irqs_disabled() || in_atomic_preempt_off() || !rcu_state.gp_kthread)
return;
+
+ /*
+ * rcu_report_qs_rdp() can only be invoked with a stable rdp and
+ * from the local CPU.
+ *
+ * The in_atomic_preempt_off() check ensures that we come here holding
+ * the last preempt_count (which will get dropped once we return to
+ * __rcu_read_unlock().
+ */
rdp = this_cpu_ptr(&rcu_data);
rdp->cpu_no_qs.b.norm = false;
rcu_report_qs_rdp(rdp);
--
2.40.1
