linux-kernel - Re: [RFC PATCH RT V4] rwsem: The return of multi-reader PI rwsems

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Date:	Fri, 18 Apr 2014 10:19:26 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	Carsten Emde <C.Emde@...dl.org>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Clark Williams <williams@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH RT V4] rwsem: The return of multi-reader PI rwsems


* Steven Rostedt <rostedt@...dmis.org> wrote:

> Changes since v3:
> 
> Clark reported that he was seeing a large latency when he added this
> patch. I tested it out on a 8 logical CPU box, and sure enough I was
> seeing it too. After spending the day debugging why, I found that I had
> a bug in rt_mutex_getprio(), where I could do:
> 
>   min(task_top_pi_waiter(task)->pi_list_entry.prio, prio)
> 
> when there was no "top_pi_waiter", which would give garbage as a
> result. This would let some tasks have higher priority than they
> should, and cause other tasks that should have high priority not run.

Would a sanity check like the one below have helped? (untested and 
such)

Thanks,

	Ingo

==========>
 kernel/locking/rtmutex_common.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/locking/rtmutex_common.h b/kernel/locking/rtmutex_common.h
index 7431a9c..36b1ce8 100644
--- a/kernel/locking/rtmutex_common.h
+++ b/kernel/locking/rtmutex_common.h
@@ -85,6 +85,7 @@ static inline int task_has_pi_waiters(struct task_struct *p)
 static inline struct rt_mutex_waiter *
 task_top_pi_waiter(struct task_struct *p)
 {
+	WARN_ON_ONCE(!p->pi_waiters_leftmost);
 	return rb_entry(p->pi_waiters_leftmost, struct rt_mutex_waiter,
 			pi_tree_entry);
 }
--
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