linux-kernel - Re: [RFC][PATCH 3/3] locking/mutex: Add lock handoff to avoid starvation

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Date:   Wed, 24 Aug 2016 15:50:10 -0400
From:   Waiman Long <waiman.long@....com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Jason Low <jason.low2@....com>,
        Ding Tianhong <dingtianhong@...wei.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <Will.Deacon@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Imre Deak <imre.deak@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Terry Rudd <terry.rudd@....com>,
        "Paul E. McKenney" <paulmck@...ibm.com>,
        Jason Low <jason.low2@...com>
Subject: Re: [RFC][PATCH 3/3] locking/mutex: Add lock handoff to avoid starvation

On 08/23/2016 04:32 PM, Peter Zijlstra wrote:
> On Tue, Aug 23, 2016 at 03:47:53PM -0400, Waiman Long wrote:
>> On 08/23/2016 08:46 AM, Peter Zijlstra wrote:
>>> N
>>> @@ -573,8 +600,14 @@ __mutex_lock_common(struct mutex *lock,
>>>   		schedule_preempt_disabled();
>>>   		spin_lock_mutex(&lock->wait_lock, flags);
>>>
>>> +		if (__mutex_owner(lock) == current)
>>> +			break;
>>> +
>>>   		if (__mutex_trylock(lock))
>>>   			break;
>>> +
>>> +		if (__mutex_waiter_is_first(lock,&waiter))
>>> +			__mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
>>>   	}
>>>   	__set_task_state(task, TASK_RUNNING);
>>>
>>>
>> You may want to think about doing some spinning while the owner is active
>> instead of going back to sleep again here.
> For sure; I just didn't bother pulling in your patches. I didn't want to
> sink in more time in case people really hated on 1/3 ;-)

I think there is race in how the handoff is being done.

CPU 0                   CPU 1                   CPU 2

-----                   -----                   -----

__mutex_lock_common:                            mutex_optimistic_spin:

   __mutex_trylock()

                         mutex_unlock:

                           if (owner&  

                              MUTEX_FLAG_HANDOFF)

                           owner&= 0x3;

                                                    __mutex_trylock();

                                                      owner = CPU2;

   __mutex_set_flag(lock,

     MUTEX_FLAG_HANDOFF)

                         __mutex_unlock_slowpath:

                         __mutex_handoff:

                           owner = CPU0;


Now both CPUs 1 and 2 think they have the lock. One way to fix that is
to check if the owner is still the original lock holder (CPU 0) before
doing the handoff, like:

--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -97,6 +97,8 @@ static void __mutex_handoff(struct mutex *lock, struct 
task_st
         for (;;) {
                 unsigned long old, new;

+               if ((owner & ~MUTEX_FLAG_ALL) != current)
+                       break;
                 new = (owner & MUTEX_FLAG_WAITERS);
                 new |= (unsigned long)task;

I also think that the MUTEX_FLAG_HANDOFF bit needs to be cleared if the list
is empty.

@@ -614,7 +633,7 @@ __mutex_lock_common(struct mutex *lock, long state, 
unsigned
         mutex_remove_waiter(lock, &waiter, task);
         /* set it to 0 if there are no waiters left: */
         if (likely(list_empty(&lock->wait_list)))
-               __mutex_clear_flag(lock, MUTEX_FLAG_WAITERS);
+               __mutex_clear_flag(lock, 
MUTEX_FLAG_WAITERS|MUTEX_FLAG_HANDOFF);

Or we should try to reset the handoff bit after the while loop exit if 
the bit is still set.

Cheers,
Longman