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

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Date:   Wed, 17 Apr 2019 13:51:24 -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 10:05 AM, Peter Zijlstra wrote:
> On Sat, Apr 13, 2019 at 01:22:54PM -0400, Waiman Long wrote:
>> @@ -650,6 +686,33 @@ __rwsem_down_read_failed_common(struct rw_semaphore *sem, int state)
>>  	struct rwsem_waiter waiter;
>>  	DEFINE_WAKE_Q(wake_q);
>>  
>> +	if (!rwsem_can_spin_on_owner(sem))
>> +		goto queue;
>> +
>> +	/*
>> +	 * Undo read bias from down_read() and do optimistic spinning.
>> +	 */
>> +	atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
>> +	adjustment = 0;
>> +	if (rwsem_optimistic_spin(sem, false)) {
>> +		unsigned long flags;
>> +
>> +		/*
>> +		 * Opportunistically wake up other readers in the wait queue.
>> +		 * It has another chance of wakeup at unlock time.
>> +		 */
>> +		if ((atomic_long_read(&sem->count) & RWSEM_FLAG_WAITERS) &&
>> +		    raw_spin_trylock_irqsave(&sem->wait_lock, flags)) {
> why trylock? Also the rest of this function uses _irq(). Having had to
> define @flags should've been a clue.
>
> You simply cnanot do down_*() with IRQs disabled.

I used trylock to avoid getting stuck in the spinlock while holding a
read lock on the rwsem.

You are right. I don't to use _irqsave version. Just the _irq version is
enough.

Thanks,
Longman