From 389abd48efe1ceacb141b2fd151263b1bc432dbc Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Wed, 21 Sep 2011 14:41:37 -0700 Subject: rcu: Avoid RCU-preempt expedited grace-period botch Because rcu_read_unlock_special() samples rcu_preempted_readers_exp(rnp) after dropping rnp->lock, the following sequence of events is possible: 1. Task A exits its RCU read-side critical section, and removes itself from the ->blkd_tasks list, releases rnp->lock, and is then preempted. Task B remains on the ->blkd_tasks list, and blocks the current expedited grace period. 2. Task B exits from its RCU read-side critical section and removes itself from the ->blkd_tasks list. Because it is the last task blocking the current expedited grace period, it ends that expedited grace period. 3. Task A resumes, and samples rcu_preempted_readers_exp(rnp) which of course indicates that nothing is blocking the nonexistent expedited grace period. Task A is again preempted. 4. Some other CPU starts an expedited grace period. There are several tasks blocking this expedited grace period queued on the same rcu_node structure that Task A was using in step 1 above. 5. Task A examines its state and incorrectly concludes that it was the last task blocking the expedited grace period on the current rcu_node structure. It therefore reports completion up the rcu_node tree. 6. The expedited grace period can then incorrectly complete before the tasks blocked on this same rcu_node structure exit their RCU read-side critical sections. Arbitrarily bad things happen. This commit therefore takes a snapshot of rcu_preempted_readers_exp(rnp) prior to dropping the lock, so that only the last task thinks that it is the last task, thus avoiding the failure scenario laid out above. Signed-off-by: Paul E. McKenney Reviewed-by: Josh Triplett diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h index 4b9b9f8..7986053 100644 --- a/kernel/rcutree_plugin.h +++ b/kernel/rcutree_plugin.h @@ -312,6 +312,7 @@ static noinline void rcu_read_unlock_special(struct task_struct *t) { int empty; int empty_exp; + int empty_exp_now; unsigned long flags; struct list_head *np; #ifdef CONFIG_RCU_BOOST @@ -382,8 +383,10 @@ static noinline void rcu_read_unlock_special(struct task_struct *t) /* * If this was the last task on the current list, and if * we aren't waiting on any CPUs, report the quiescent state. - * Note that rcu_report_unblock_qs_rnp() releases rnp->lock. + * Note that rcu_report_unblock_qs_rnp() releases rnp->lock, + * so we must take a snapshot of the expedited state. */ + empty_exp_now = !rcu_preempted_readers_exp(rnp); if (!empty && !rcu_preempt_blocked_readers_cgp(rnp)) { trace_rcu_quiescent_state_report("preempt_rcu", rnp->gpnum, @@ -406,7 +409,7 @@ static noinline void rcu_read_unlock_special(struct task_struct *t) * If this was the last task on the expedited lists, * then we need to report up the rcu_node hierarchy. */ - if (!empty_exp && !rcu_preempted_readers_exp(rnp)) + if (!empty_exp && empty_exp_now) rcu_report_exp_rnp(&rcu_preempt_state, rnp); } else { local_irq_restore(flags); -- cgit v0.10.2