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From: Thomas Gleixner <tglx@linutronix.de>
To: Yimin Deng <yimin11.deng@gmail.com>
Cc: linux-rt-users@vger.kernel.org
Subject: Re: kernel BUG at kernel/rtmutex_common.h:75
Date: Fri, 6 Nov 2015 15:41:16 +0100 (CET)	[thread overview]
Message-ID: <alpine.DEB.2.11.1511061215400.4032@nanos> (raw)
In-Reply-To: <CAAh1qt=DCL9aUXNxanP5BKtiPp3m+qj4yB+gDohhXPVFCxWwzg@mail.gmail.com>

[-- Attachment #1: Type: TEXT/PLAIN, Size: 1857 bytes --]

B1;2802;0cOn Wed, 4 Nov 2015, Yimin Deng wrote:
> It seems that the purpose to call the remove_waiter() is to remove the
> waiter added by “plist_add(&waiter->list_entry, &lock->wait_list);” in
> the task_blocks_on_rt_mutex(). But in the scenario above there's no
> waiter on the lock yet and
> the waiter has not been added into the wait list of the lock in the
> task_blocks_on_rt_mutex() due to the failure “-EAGAIN”. So it reported
> kernel BUG in the rt_mutex_top_waiter().
> 
> I modified it as below and the issue seems disappear.
> - if (unlikely(ret))
> + if (unlikely(ret && (-EAGAIN != ret)))
>        remove_waiter(lock, waiter);
> 
> Could the scenario above be possible? If so, how to resolve this issue?
> Thanks!

Yes it is possible. Nice detective work!

Your solution is correct, but actually it's not sufficient, because we
have another possibility to return early without being queued
(-EDEADLOCK). Find the full solution below.

Thanks for tracking that down!

       tglx
---
diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
index 7601c1332a88..0e6505d5ce4a 100644
--- a/kernel/rtmutex.c
+++ b/kernel/rtmutex.c
@@ -1003,11 +1003,18 @@ static void wakeup_next_waiter(struct rt_mutex *lock)
 static void remove_waiter(struct rt_mutex *lock,
 			  struct rt_mutex_waiter *waiter)
 {
-	bool is_top_waiter = (waiter == rt_mutex_top_waiter(lock));
 	struct task_struct *owner = rt_mutex_owner(lock);
 	struct rt_mutex *next_lock = NULL;
+	bool is_top_waiter = false;
 	unsigned long flags;
 
+	/*
+	 * @waiter might be not queued when task_blocks_on_rt_mutex()
+	 * returned early so @lock might not have any waiters.
+	 */
+	if (rt_mutex_has_waiters())
+		is_top_waiter = (waiter == rt_mutex_top_waiter(lock));
+
 	raw_spin_lock_irqsave(&current->pi_lock, flags);
 	rt_mutex_dequeue(lock, waiter);
 	current->pi_blocked_on = NULL;


  reply	other threads:[~2015-11-06 14:41 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-11-04 14:35 Yimin Deng
2015-11-06 14:41 ` Thomas Gleixner [this message]
2015-11-07 18:09   ` Thomas Gleixner
2015-11-08  3:31     ` Yimin Deng
2018-03-12 14:28 [PATCH] kernel/rtmutex: Handle non enqueued waiters gracefully in remove_waiter() Sebastian Andrzej Siewior
2018-03-16 12:20 ` Peter Zijlstra
2018-03-19 15:11   ` Thomas Gleixner
2018-03-27 12:14     ` [PATCH v2] locking/rtmutex: " Sebastian Andrzej Siewior
2018-03-28 21:07       ` [tip:locking/core] " tip-bot for Peter Zijlstra

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