linux-kernel - Re: [patch 2/2] rtmutex: Avoid pointless requeueing in the deadlock detection chain walk

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Message-ID: <20140515130435.24132d1b@gandalf.local.home>
Date:	Thu, 15 May 2014 13:04:35 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [patch 2/2] rtmutex: Avoid pointless requeueing in the deadlock
 detection chain walk

On Wed, 14 May 2014 20:03:27 -0000
Thomas Gleixner <tglx@...utronix.de> wrote:

> In case the dead lock detector is enabled we follow the lock chain to
> the end in rt_mutex_adjust_prio_chain, even if we could stop earlier
> due to the priority/waiter constellation.

I'm assuming that we want to detect deadlocks for all futex calls
even when CONFIG_DEBUG_RT_MUTEXES is set?

In kernel/locking/rtmutex_common.h:

#ifdef CONFIG_DEBUG_RT_MUTEXES
# include "rtmutex-debug.h"
#else
# include "rtmutex.h"
#endif

In kernel/locking/rtmutex.h:

#define debug_rt_mutex_detect_deadlock(w,d)             (d)

In kernel/locking/rtmutex.h:

static inline int debug_rt_mutex_detect_deadlock(struct rt_mutex_waiter *waiter,
                                                 int detect)
{
        return (waiter != NULL);
}

Shouldn't that be: return detect || waiter != NULL;

?

I know this a separate issue from this patch series, but it's
something that I just noticed.

> 
> But once we are not longer the top priority waiter in a certain step

"we are no longer the top"

> or the task holding the lock has already the same priority then there
> is no point in dequeing and enqueing along the lock chain as there is
> no change at all.
> 
> So stop the queueing at this point.

I'll continue reviewing the patch.

-- Steve
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