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Subject[PATCH] rcu: exp: Protect all sync_rcu_preempt_exp_done() with rcu_node lock
Date
Currently some callsites of sync_rcu_preempt_exp_done() are not called
with the corresponding rcu_node's ->lock held, which could introduces
bugs as per Paul:

o CPU 0 in sync_rcu_preempt_exp_done() reads ->exp_tasks and
sees that it is NULL.

o CPU 1 blocks within an RCU read-side critical section, so
it enqueues the task and points ->exp_tasks at it and
clears CPU 1's bit in ->expmask.

o All other CPUs clear their bits in ->expmask.

o CPU 0 reads ->expmask, sees that it is zero, so incorrectly
concludes that all quiescent states have completed, despite
the fact that ->exp_tasks is non-NULL.

To fix this, sync_rcu_preempt_exp_unlocked() is introduced to replace
lockless callsites of sync_rcu_preempt_exp_done().

Further, a lockdep annotation is added into sync_rcu_preempt_exp_done()
to prevent mis-use in the future.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
kernel/rcu/tree_exp.h | 30 +++++++++++++++++++++++++++---
1 file changed, 27 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 2fd882b08b7c..150839a2c00a 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -20,6 +20,8 @@
* Authors: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
*/

+#include <linux/lockdep.h>
+
/*
* Record the start of an expedited grace period.
*/
@@ -158,10 +160,30 @@ static void __maybe_unused sync_exp_reset_tree(struct rcu_state *rsp)
*/
static bool sync_rcu_preempt_exp_done(struct rcu_node *rnp)
{
+ lockdep_assert_held(&rnp->lock);
+
return rnp->exp_tasks == NULL &&
READ_ONCE(rnp->expmask) == 0;
}

+/*
+ * Like sync_rcu_preempt_exp_done(), but this function assumes the caller
+ * doesn't hold the rcu_node's ->lock, and will acquire and release the lock
+ * itself
+ */
+static bool sync_rcu_preempt_exp_done_unlocked(struct rcu_node *rnp)
+{
+ unsigned long flags;
+ bool ret;
+
+ raw_spin_lock_irqsave_rcu_node(rnp, flags);
+ ret = sync_rcu_preempt_exp_done(rnp);
+ raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+
+ return ret;
+}
+
+
/*
* Report the exit from RCU read-side critical section for the last task
* that queued itself during or before the current expedited preemptible-RCU
@@ -498,9 +520,9 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
for (;;) {
ret = swait_event_timeout(
rsp->expedited_wq,
- sync_rcu_preempt_exp_done(rnp_root),
+ sync_rcu_preempt_exp_done_unlocked(rnp_root),
jiffies_stall);
- if (ret > 0 || sync_rcu_preempt_exp_done(rnp_root))
+ if (ret > 0 || sync_rcu_preempt_exp_done_unlocked(rnp_root))
return;
WARN_ON(ret < 0); /* workqueues should not be signaled. */
if (rcu_cpu_stall_suppress)
@@ -533,8 +555,10 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
rcu_for_each_node_breadth_first(rsp, rnp) {
if (rnp == rnp_root)
continue; /* printed unconditionally */
- if (sync_rcu_preempt_exp_done(rnp))
+
+ if (sync_rcu_preempt_exp_done_unlocked(rnp))
continue;
+
pr_cont(" l=%u:%d-%d:%#lx/%c",
rnp->level, rnp->grplo, rnp->grphi,
rnp->expmask,
--
2.16.2
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