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rcu: Revert "Allow post-unlock reference for rt_mutex" to avoid priority-inversion

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The patch dfeb9765ce ("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 27e35715df ("rtmutex: Plug slow unlock race").
This fix made the previous patch (dfeb9765ce) 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>
This commit is contained in:
Lai Jiangshan 2014-11-18 16:30:01 +08:00 committed by Paul E. McKenney
parent 3ba4d0e09b
commit abaf3f9d27
2 changed files with 1 additions and 12 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
kernel/rcu/tree.h
View File

@ -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. */

8
kernel/rcu/tree_plugin.h
View File

@ -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;
}