[PATCH tip/core/rcu 12/14] rcu: Revert "Allow post-unlock reference for rt_mutex" to avoid priority-inversion

From: Paul E. McKenney
Date: Wed Jan 07 2015 - 12:35:51 EST


From: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>

The patch dfeb9765ce3c ("Allow post-unlock reference for rt_mutex")
ensured rcu-boost safe even the rt_mutex has post-unlock reference.

But rt_mutex allowing post-unlock reference is definitely a bug and it was
fixed by the commit 27e35715df54 ("rtmutex: Plug slow unlock race").
This fix made the previous patch (dfeb9765ce3c) useless.

And even worse, the priority-inversion introduced by the the previous
patch still exists.

rcu_read_unlock_special() {
rt_mutex_unlock(&rnp->boost_mtx);
/* Priority-Inversion:
* the current task had been deboosted and preempted as a low
* priority task immediately, it could wait long before reschedule in,
* and the rcu-booster also waits on this low priority task and sleeps.
* This priority-inversion makes rcu-booster can't work
* as expected.
*/
complete(&rnp->boost_completion);
}

Just revert the patch to avoid it.

Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
kernel/rcu/tree.h | 5 -----
kernel/rcu/tree_plugin.h | 8 +-------
2 files changed, 1 insertion(+), 12 deletions(-)

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 883ebc8e2b6e..95356477d560 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -172,11 +172,6 @@ struct rcu_node {
/* queued on this rcu_node structure that */
/* are blocking the current grace period, */
/* there can be no such task. */
- struct completion boost_completion;
- /* Used to ensure that the rt_mutex used */
- /* to carry out the boosting is fully */
- /* released with no future boostee accesses */
- /* before that rt_mutex is re-initialized. */
struct rt_mutex boost_mtx;
/* Used only for the priority-boosting */
/* side effect, not as a lock. */
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 1fac68220999..625e26040e6b 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -429,10 +429,8 @@ void rcu_read_unlock_special(struct task_struct *t)

#ifdef CONFIG_RCU_BOOST
/* Unboost if we were boosted. */
- if (drop_boost_mutex) {
+ if (drop_boost_mutex)
rt_mutex_unlock(&rnp->boost_mtx);
- complete(&rnp->boost_completion);
- }
#endif /* #ifdef CONFIG_RCU_BOOST */

/*
@@ -1081,15 +1079,11 @@ static int rcu_boost(struct rcu_node *rnp)
*/
t = container_of(tb, struct task_struct, rcu_node_entry);
rt_mutex_init_proxy_locked(&rnp->boost_mtx, t);
- init_completion(&rnp->boost_completion);
raw_spin_unlock_irqrestore(&rnp->lock, flags);
/* Lock only for side effect: boosts task t's priority. */
rt_mutex_lock(&rnp->boost_mtx);
rt_mutex_unlock(&rnp->boost_mtx); /* Then keep lockdep happy. */

- /* Wait for boostee to be done w/boost_mtx before reinitializing. */
- wait_for_completion(&rnp->boost_completion);
-
return ACCESS_ONCE(rnp->exp_tasks) != NULL ||
ACCESS_ONCE(rnp->boost_tasks) != NULL;
}
--
1.8.1.5

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