linux-kernel - Re: [PATCH -next v4] blk-mq: fix tag

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Date:   Wed, 12 Jan 2022 12:51:13 +0000
From:   John Garry <john.garry@...wei.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        QiuLaibin <qiulaibin@...wei.com>
CC:     <axboe@...nel.dk>, <ming.lei@...hat.com>,
        <martin.petersen@...cle.com>, <hare@...e.de>,
        <johannes.thumshirn@....com>, <bvanassche@....org>,
        <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next v4] blk-mq: fix tag_get wait task can't be awakened

On 12/01/2022 12:30, Andy Shevchenko wrote:
>>>> +		if (test_bit(QUEUE_FLAG_HCTX_ACTIVE, &q->queue_flags) ||
>>>> +		    test_and_set_bit(QUEUE_FLAG_HCTX_ACTIVE, &q->queue_flags)) {
>>> Whoever wrote this code did too much defensive programming, because the first
>>> conditional doesn't make much sense here. Am I right?
>>>
>> I think because this judgement is in the general IO process, there are also
>> some performance considerations here.
> I didn't buy this. Is there any better argument why you need redundant
> test_bit() call?
> 

I think that the idea is that test_bit() is fast and test_and_set_bit() 
is slow; as such, if we generally expect the bit to be set, then there 
is no need to do the slower test_and_set_bit() always.