linux-kernel - Re: [PATCH -v5][RFC]: mutex: implement adaptive spinning

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Date:	Wed, 07 Jan 2009 22:51:55 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>, paulmck@...ux.vnet.ibm.com,
	Gregory Haskins <ghaskins@...ell.com>,
	Ingo Molnar <mingo@...e.hu>, Matthew Wilcox <matthew@....cx>,
	Andi Kleen <andi@...stfloor.org>,
	Chris Mason <chris.mason@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nick Piggin <npiggin@...e.de>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>
Subject: Re: [PATCH -v5][RFC]: mutex: implement adaptive spinning

On Wed, 2009-01-07 at 12:55 -0800, Linus Torvalds wrote:

> 	/*
> 	 * Look out! "thread" is an entirely speculative pointer
> 	 * access and not reliable.
> 	 */
> 	void loop_while_oncpu(struct mutex *lock, struct thread_struct *thread)
> 	{
> 		for (;;) {
> 			unsigned cpu;
> 			struct runqueue *rq;
> 
> 			if (lock->owner != thread)
> 				break;
> 
> 			/*
> 			 * Need to access the cpu field knowing that
> 			 * DEBUG_PAGEALLOC could have unmapped it if
> 			 * the mutex owner just released it and exited.
> 			 */
> 			if (__get_user(cpu, &thread->cpu))
> 				break;
> 
> 			/*
> 			 * Even if the access succeeded (likely case),
> 			 * the cpu field may no longer be valid. FIXME:
> 			 * this needs to validate that we can do a
> 			 * get_cpu() and that we have the percpu area.
> 			 */
> 			if (cpu >= NR_CPUS)
> 				break;
> 
> 			if (!cpu_online(cpu))
> 				break;
> 
> 			/*
> 			 * Is that thread really running on that cpu?
> 			 */
> 			rq = cpu_rq(cpu);
> 			if (task_thread_info(rq->curr) != thread)
> 				break;
> 
> 			cpu_relax();
> 		}
> 	}

Do we really have to re-do all that code every loop?

        void loop_while_oncpu(struct mutex *lock, struct thread_struct *thread)
        {
                unsigned cpu;
                struct runqueue *rq;

                /*
                 * Need to access the cpu field knowing that
                 * DEBUG_PAGEALLOC could have unmapped it if
                 * the mutex owner just released it and exited.
                 */
                if (__get_user(cpu, &thread->cpu))
                        break;

                /*
                 * Even if the access succeeded (likely case),
                 * the cpu field may no longer be valid. FIXME:
                 * this needs to validate that we can do a
                 * get_cpu() and that we have the percpu area.
                 */
                if (cpu >= NR_CPUS)
                        break;

                if (!cpu_online(cpu))
                        break;

                rq = cpu_rq(cpu);

                for (;;) {
                        if (lock->owner != thread)
                                break;

                        /*
                         * Is that thread really running on that cpu?
                         */
                        if (task_thread_info(rq->curr) != thread)
                                break;

                        cpu_relax();
                }
        }

Also, it would still need to do the funny:

 l_owner = ACCESS_ONCE(lock->owner)
 if (l_owner && l_owner != thread)
   break;

thing, to handle the premature non-atomic lock->owner tracking.
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