From 5fd4dc068c4ded1339180dbcd1a99e15b1c0a728 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Fri, 10 Aug 2012 16:00:11 -0700 Subject: [PATCH] rcu: Avoid rcu_print_detail_task_stall_rnp() segfault The rcu_print_detail_task_stall_rnp() function invokes rcu_preempt_blocked_readers_cgp() to verify that there are some preempted RCU readers blocking the current grace period outside of the protection of the rcu_node structure's ->lock. This means that the last blocked reader might exit its RCU read-side critical section and remove itself from the ->blkd_tasks list before the ->lock is acquired, resulting in a segmentation fault when the subsequent code attempts to dereference the now-NULL gp_tasks pointer. This commit therefore moves the test under the lock. This will not have measurable effect on lock contention because this code is invoked only when printing RCU CPU stall warnings, in other words, in the common case, never. Signed-off-by: Paul E. McKenney --- kernel/rcutree_plugin.h | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h index 70b33bf780f0..df47014e129d 100644 --- a/kernel/rcutree_plugin.h +++ b/kernel/rcutree_plugin.h @@ -422,9 +422,11 @@ static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp) unsigned long flags; struct task_struct *t; - if (!rcu_preempt_blocked_readers_cgp(rnp)) - return; raw_spin_lock_irqsave(&rnp->lock, flags); + if (!rcu_preempt_blocked_readers_cgp(rnp)) { + raw_spin_unlock_irqrestore(&rnp->lock, flags); + return; + } t = list_entry(rnp->gp_tasks, struct task_struct, rcu_node_entry); list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) -- 2.20.1