[PATCH 01/11] rcu: avoid leaking exp_deferred_qs into next GP

Lai Jiangshan
2019-10-31T10:08:40Z

If exp_deferred_qs is incorrectly set and leaked to the next
exp GP, it may cause the next GP to be incorrectly prematurely
completed.

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
kernel/rcu/tree_exp.h | 23 ++++++++++++++---------
1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index a0e1e51c51c2..6dec21909b30 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -603,6 +603,18 @@ static void rcu_exp_handler(void *unused)
struct rcu_node *rnp = rdp->mynode;
struct task_struct *t = current;

+ /*
+ * Note that there is a large group of race conditions that
+ * can have caused this quiescent state to already have been
+ * reported, so we really do need to check ->expmask first.
+ */
+ raw_spin_lock_irqsave_rcu_node(rnp, flags);
+ if (!(rnp->expmask & rdp->grpmask)) {
+ raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+ return;
+ }
+ raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+
/*
* First, the common case of not being in an RCU read-side
* critical section. If also enabled or idle, immediately
@@ -628,17 +640,10 @@ static void rcu_exp_handler(void *unused)
* a future context switch. Either way, if the expedited
* grace period is still waiting on this CPU, set ->deferred_qs
* so that the eventual quiescent state will be reported.
- * Note that there is a large group of race conditions that
- * can have caused this quiescent state to already have been
- * reported, so we really do need to check ->expmask.
*/
if (t->rcu_read_lock_nesting > 0) {
- raw_spin_lock_irqsave_rcu_node(rnp, flags);
- if (rnp->expmask & rdp->grpmask) {
- rdp->exp_deferred_qs = true;
- t->rcu_read_unlock_special.b.exp_hint = true;
- }
- raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+ rdp->exp_deferred_qs = true;
+ WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
return;
}

--
2.20.1