[PATCH rcu 4/9] rcu: handle unstable rdp in rcu_read_unlock_strict()

[Paul E. McKenney]

From: Ankur Arora <ankur.a.ar...@oracle.com>

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 <frede...@kernel.org>
Signed-off-by: Ankur Arora <ankur.a.ar...@oracle.com>
Reviewed-by: Frederic Weisbecker <frede...@kernel.org>
Signed-off-by: Paul E. McKenney <paul...@kernel.org>
---
 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 c6c4aee47df2c..9b05db8ff0619 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 bb7ca6eb9ef04..b025e95ede053 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