linux-kernel - Re: [RFC 3/3] mutex: When there is no owner, stop spinning after too many tries

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Date:	Wed, 15 Jan 2014 22:46:17 -0800
From:	Jason Low <jason.low2@...com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Waiman Long <Waiman.Long@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Davidlohr Bueso <davidlohr@...com>,
	Peter Anvin <hpa@...or.com>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [RFC 3/3] mutex: When there is no owner, stop spinning after
 too many tries

On Thu, 2014-01-16 at 10:14 +0700, Linus Torvalds wrote:
> On Thu, Jan 16, 2014 at 9:45 AM, Jason Low <jason.low2@...com> wrote:
> >
> > Any comments on the below change which unlocks the mutex before taking
> > the lock->wait_lock to wake up a waiter? Thanks.
> 
> Hmm. Doesn't that mean that a new lock owner can come in *before*
> you've called debug_mutex_unlock and the lockdep stuff, and get the
> lock? And then debug_mutex_lock() will be called *before* the unlocker
> called debug_mutex_unlock(), which I'm sure confuses things.

If obtaining the wait_lock for debug_mutex_unlock is the issue, then
perhaps we can address that by taking care of
#ifdef CONFIG_DEBUG_MUTEXES. In the CONFIG_DEBUG_MUTEXES case, we can
take the wait_lock first, and in the regular case, take the wait_lock
after releasing the mutex.

#ifdef CONFIG_DEBUG_MUTEXES
	spin_lock_mutex(&lock->wait_lock, flags);
#endif

        mutex_release(&lock->dep_map, nested, _RET_IP_);
        debug_mutex_unlock(lock);
        if (__mutex_slowpath_needs_to_unlock())
                atomic_set(&lock->count, 1);

#ifndef CONFIG_DEBUG_MUTEXES
	spin_lock_mutex(&lock->wait_lock, flags);
#endif

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