rcu: Revert "Allow post-unlock reference for rt_mutex" to avoid priority-inversion (abaf3f9d) · Commits · 戴 / test

The patch ("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 ("rtmutex: Plug slow unlock race"). This fix made the previous patch () 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@linutronix.de> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Peter Zijlstra <peterz@infradead.org> Signed-off-by:

Lai Jiangshan <laijs@cn.fujitsu.com> Signed-off-by:

Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.h

+0 −5

Original line number	Diff line number	Diff line
		@@ -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. */

kernel/rcu/tree_plugin.h

+1 −7

Original line number	Diff line number	Diff line
		@@ -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;
		}

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