linux-kernel - Re: [PATCH v4 11/16] locking/rwsem: Enable readers spinning on writer

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Date:   Wed, 17 Apr 2019 13:34:01 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        huang ying <huang.ying.caritas@...il.com>
Subject: Re: [PATCH v4 11/16] locking/rwsem: Enable readers spinning on writer

On 04/17/2019 09:56 AM, Peter Zijlstra wrote:
> On Sat, Apr 13, 2019 at 01:22:54PM -0400, Waiman Long wrote:
>> @@ -549,7 +582,7 @@ static noinline enum owner_state rwsem_spin_on_owner(struct rw_semaphore *sem)
>>  	return !owner ? OWNER_NULL : OWNER_READER;
>>  }
>>  
>> -static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
>> +static bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock)
>>  {
>>  	bool taken = false;
>>  	bool is_rt_task = rt_task(current);
>> @@ -558,9 +591,6 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
>>  	preempt_disable();
>>  
>>  	/* sem->wait_lock should not be held when doing optimistic spinning */
>> -	if (!rwsem_can_spin_on_owner(sem))
>> -		goto done;
>> -
>>  	if (!osq_lock(&sem->osq))
>>  		goto done;
>>  
>> @@ -580,10 +610,11 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
>>  		/*
>>  		 * Try to acquire the lock
>>  		 */
>> -		if (rwsem_try_write_lock_unqueued(sem)) {
>> -			taken = true;
>> +		taken = wlock ? rwsem_try_write_lock_unqueued(sem)
>> +			      : rwsem_try_read_lock_unqueued(sem);
>> +
>> +		if (taken)
>>  			break;
>> -		}
>>  
>>  		/*
>>  		 * An RT task cannot do optimistic spinning if it cannot
> Alternatively you pass the trylock function as an argument:
>
> static bool rwsem_optimistic_spin(struct rw_semaphore *sem,
> 				  bool (*trylock)(struct rw_semaphore *sem))
> {
> 	...
> 		if (trylock(sem)) {
> 			taken = true;
> 			goto unlock;
> 		}
> 	...
> }
>
With retpoline, an indirect function call will be slower.

Cheers,
Longman