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Subject: Re: [PATCH] blk-wbt: get back the missed wakeup from __wbt_done
To:     Jens Axboe <axboe@kernel.dk>, Anchal Agarwal <anchalag@amzn.com>
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        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
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From:   "jianchao.wang" <jianchao.w.wang@oracle.com>
Message-ID: <3876c36c-fec6-6586-435f-dc177dc38c76@oracle.com>
Date:   Mon, 27 Aug 2018 14:15:07 +0800
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On 08/27/2018 11:52 AM, jianchao.wang wrote:
> Hi Jens
> 
> On 08/25/2018 11:41 PM, Jens Axboe wrote:
>>  	do {
>> -		set_current_state(TASK_UNINTERRUPTIBLE);
>> +		if (test_bit(0, &data.flags))
>> +			break;
>>  
>> -		if (!has_sleeper && rq_wait_inc_below(rqw, get_limit(rwb, rw)))
>> +		WARN_ON_ONCE(list_empty(&data.wq.entry));
>> +
>> +		if (!has_sleeper &&
>> +		    rq_wait_inc_below(rqw, get_limit(rwb, rw))) {
>> +			finish_wait(&rqw->wait, &data.wq);
>> +
>> +			/*
>> +			 * We raced with wbt_wake_function() getting a token,
>> +			 * which means we now have two. Put ours and wake
>> +			 * anyone else potentially waiting for one.
>> +			 */
>> +			if (test_bit(0, &data.flags))
>> +				wbt_rqw_done(rwb, rqw, wb_acct);
>>  			break;
> 
> Just use 'bool' variable should be OK 
> After finish_wait, no one could race with us here.
> 
>> +		}
>>  
>>  		if (lock) {
>>  			spin_unlock_irq(lock);
>> @@ -511,11 +569,11 @@ static void __wbt_wait(struct rq_wb *rwb, enum wbt_flags wb_acct,
>>  			spin_lock_irq(lock);
>>  		} else
>>  			io_schedule();
>> +
>>  		has_sleeper = false;
>>  	} while (1);
> 
> I cannot get the point of "since we can't rely on just being woken from the ->func handler
> we set".
> Do you mean there could be someone else could wake up this task ?
> 

If we do need a recheck after the io_schedule, we could do as following:

static void __wbt_wait(struct rq_wb *rwb, enum wbt_flags wb_acct,
		       unsigned long rw, spinlock_t *lock)
	__releases(lock)
	__acquires(lock)
{
	struct rq_wait *rqw = get_rq_wait(rwb, wb_acct);
	struct wbt_wait_data data = {
		.wq = {
			.func	= wbt_wake_function,
			.entry	= LIST_HEAD_INIT(data.wq.entry),
		},
		.curr = current,
		.rwb = rwb,
		.rqw = rqw,
		.rw = rw,
	};
	bool has_sleeper;
	bool got = false;

retry:
	has_sleeper = wq_has_sleeper(&rqw->wait);
	if (!has_sleeper && rq_wait_inc_below(rqw, get_limit(rwb, rw)))
		return;

	prepare_to_wait_exclusive(&rqw->wait, &data.wq, TASK_UNINTERRUPTIBLE);

	if (!has_sleeper && rq_wait_inc_below(rqw, get_limit(rwb, rw))) {
		got = true;
		goto out;
	}

	if (lock) {
		spin_unlock_irq(lock);
		io_schedule();
		spin_lock_irq(lock);
	} else
		io_schedule();

out:
	finish_wait(&rqw->wait, &data.wq);

	/*
	 * We raced with wbt_wake_function() getting a token,
	 * which means we now have two. Put ours and wake
	 * anyone else potentially waiting for one.
	 */
	if (data.got && got)
		wbt_rqw_done(rwb, rqw, wb_acct);
	else if (!data.got && !got)
		goto retry;

Thanks
Jianchao