linux-kernel - Re: [PATCH-queue/locking/core] locking/mutex: Unify yield_to_waiter & waiter

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Date:   Fri, 19 Aug 2016 11:25:04 -0400
From:   Waiman Long <waiman.long@....com>
To:     Jason Low <jason.low2@....com>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ding Tianhong <dingtianhong@...wei.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        "Paul E. McKenney" <paulmck@...ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <Will.Deacon@....com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Imre Deak <imre.deak@...el.com>, <jason.low2@...com>
Subject: Re: [PATCH-queue/locking/core] locking/mutex: Unify yield_to_waiter
 & waiter_spinning

On 08/18/2016 09:28 PM, Jason Low wrote:
> On Thu, 2016-08-18 at 21:15 -0400, Waiman Long wrote:
>> Both waiter_spinning and yield_to_waiter are used for somewhat similar
>> purpose. The waiter_spinning flag is used in CONFIG_MUTEX_SPIN_ON_OWNER
>> to make optimistic spinner yield to spinning waiter; whereas the
>> yield_to_waiter is used in !CONFIG_MUTEX_SPIN_ON_OWNER to make new
>> incoming mutex locker to yield to the sleeping waiter.
>>
>> This patch unifies these 2 flags into a single yield_to_waiter
>> flag that is used in both CONFIG_MUTEX_SPIN_ON_OWNER and
>> !CONFIG_MUTEX_SPIN_ON_OWNER codes.
>>
>> Signed-off-by: Waiman Long<Waiman.Long@....com>
>> ---
>>   include/linux/mutex.h  |    8 ++++++--
>>   kernel/locking/mutex.c |   12 ++++++------
>>   2 files changed, 12 insertions(+), 8 deletions(-)
>>
>> diff --git a/include/linux/mutex.h b/include/linux/mutex.h
>> index 988c020..2b3dcdb 100644
>> --- a/include/linux/mutex.h
>> +++ b/include/linux/mutex.h
>> @@ -57,8 +57,12 @@ struct mutex {
>>   #endif
>>   #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
>>   	struct optimistic_spin_queue osq; /* Spinner MCS lock */
>> -	int waiter_spinning;
>> -#elif defined(CONFIG_SMP)
>> +#endif
>> +#ifdef CONFIG_SMP
>> +	/*
>> +	 * Used by both CONFIG_MUTEX_SPIN_ON_OWNER (depends on CONFIG_SMP)&
>> +	 * !CONFIG_MUTEX_SPIN_ON_OWNER codes.
>> +	 */
>>   	int yield_to_waiter;
>>   #endif
>>   #ifdef CONFIG_DEBUG_MUTEXES
>> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
>> index c4261fa..7218835 100644
>> --- a/kernel/locking/mutex.c
>> +++ b/kernel/locking/mutex.c
>> @@ -55,8 +55,8 @@ __mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
>>   	mutex_clear_owner(lock);
>>   #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
>>   	osq_lock_init(&lock->osq);
>> -	lock->waiter_spinning = false;
>> -#elif defined(CONFIG_SMP)
>> +#endif
>> +#ifdef CONFIG_SMP
>>   	lock->yield_to_waiter = false;
>>   #endif
>>
>> @@ -351,7 +351,7 @@ static bool mutex_optimistic_spin(struct mutex *lock,
>>   		 * Turn on the waiter spinning flag to discourage the spinner
>>   		 * from getting the lock.
> Might want to update this comment to "Turn on the yield to waiter flag
> to discourage optimistic spinners from stealing the lock."
>
> Besides that:
>
> Acked-by: Jason Low<jason.low2@....com>
>

You are right. I should have made that comment change. As you have a new 
patch outstanding, I will wait a bit until it stabilizes before I update 
my patch.

Cheers,
Longman