linux-kernel - Re: [PATCH v3 1/5] blk-mq-sched: introduce high level elevator lock

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Message-ID: <5974ead5-2ef1-4887-91f5-422c87c30273@kernel.org>
Date: Mon, 11 Aug 2025 13:34:47 +0900
From: Damien Le Moal <dlemoal@...nel.org>
To: Yu Kuai <yukuai1@...weicloud.com>, hare@...e.de, jack@...e.cz,
 bvanassche@....org, tj@...nel.org, josef@...icpanda.com, axboe@...nel.dk
Cc: cgroups@...r.kernel.org, linux-block@...r.kernel.org,
 linux-kernel@...r.kernel.org, yi.zhang@...wei.com, yangerkun@...wei.com,
 johnny.chenyi@...wei.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH v3 1/5] blk-mq-sched: introduce high level elevator lock

On 8/11/25 13:25, Yu Kuai wrote:
> Hi,
> 
> 在 2025/08/11 11:53, Damien Le Moal 写道:
>> On 8/11/25 10:01, Yu Kuai wrote:
>>>>> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
>>>>> index 55a0fd105147..1a2da5edbe13 100644
>>>>> --- a/block/blk-mq-sched.c
>>>>> +++ b/block/blk-mq-sched.c
>>>>> @@ -113,7 +113,14 @@ static int __blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
>>>>>    		if (budget_token < 0)
>>>>>    			break;
>>>>>    
>>>>> -		rq = e->type->ops.dispatch_request(hctx);
>>>>> +		if (blk_queue_sq_sched(q)) {
>>>>> +			elevator_lock(e);
>>>>> +			rq = e->type->ops.dispatch_request(hctx);
>>>>> +			elevator_unlock(e);
>>>>
>>>> I do not think this is safe for bfq since bfq uses the irqsave/irqrestore spin
>>>> lock variant. If it is safe, this needs a big comment block explaining why
>>>> and/or the rules regarding the scheduler use of this lock.
>>>
>>> It's correct, however, this patch doesn't change bfq yet, and it's like:
>>>
>>> elevator_lock
>>> spin_lock_irq(&bfqd->lock)
>>> spin_unlock_irq(&bfqd->lock)
>>> elevator_unlock
>>>
>>> Patch 3 remove bfqd->lock and convert this to:
>>>
>>> elevator_lock_irq
>>> elevator_unlock_irq.
>>
>> I do not understand. Since q->elevator->lock is already taken here, without IRQ
>> disabled, how can bfq_dispatch_request method again take this same lock with IRQ
>> disabled ? That cannot possibly work.
> 
> Looks like there is still misunderstanding somehow :( After patch 3,
> bfq_dispatch_work doesn't grab any lock, elevator lock is held before
> calling into dispatch method.
> 
> Before:
> 
> elevator_lock
> bfq_dispatch_request
>   spin_lock_irq(&bfqd->lock)
>   spin_unlock_irq(&bfqd->lock)
> elevator_unlock
> 
> After:
> elevator_lock_irq
> bfq_dispatch_request
> elevator_unlock_irq

Ah, yes, I see it now.

But that is a nasty change that affects *all* schedulers, even those that do not
need to disable IRQs because they are not using the lock in their completion
path, e.g. mq-deadline. So I do not think that is acceptable.

-- 
Damien Le Moal
Western Digital Research