Re: [dm-devel] [PATCH 2/3] scsi: make sure that request queue queiesce and unquiesce balanced

[Jens Axboe <axboe@kernel.dk>]

On 11/2/21 8:47 AM, James Bottomley wrote:
> On Tue, 2021-11-02 at 08:41 -0600, Jens Axboe wrote:
>> On 11/2/21 8:36 AM, Jens Axboe wrote:
>>> On 11/2/21 8:33 AM, James Bottomley wrote:
>>>> On Tue, 2021-11-02 at 06:59 -0600, Jens Axboe wrote:
>>>>> On 11/1/21 7:43 PM, James Bottomley wrote:
>>>>>> On Thu, 2021-10-21 at 22:59 +0800, Ming Lei wrote:
>>>>>>> For fixing queue quiesce race between driver and block
>>>>>>> layer(elevator switch, update nr_requests, ...), we need to
>>>>>>> support concurrent quiesce and unquiesce, which requires
>>>>>>> the two
>>>>>>> call balanced.
>>>>>>>
>>>>>>> It isn't easy to audit that in all scsi drivers, especially
>>>>>>> the two may be called from different contexts, so do it in
>>>>>>> scsi core with one per-device bit flag & global spinlock,
>>>>>>> basically zero cost since request queue quiesce is seldom
>>>>>>> triggered.
>>>>>>>
>>>>>>> Reported-by: Yi Zhang <yi.zhang@redhat.com>
>>>>>>> Fixes: e70feb8b3e68 ("blk-mq: support concurrent queue
>>>>>>> quiesce/unquiesce")
>>>>>>> Signed-off-by: Ming Lei <ming.lei@redhat.com>
>>>>>>> ---
>>>>>>>  drivers/scsi/scsi_lib.c    | 45
>>>>>>> ++++++++++++++++++++++++++++++
>>>>>>> ----
>>>>>>> ----
>>>>>>>  include/scsi/scsi_device.h |  1 +
>>>>>>>  2 files changed, 37 insertions(+), 9 deletions(-)
>>>>>>>
>>>>>>> diff --git a/drivers/scsi/scsi_lib.c
>>>>>>> b/drivers/scsi/scsi_lib.c
>>>>>>> index 51fcd46be265..414f4daf8005 100644
>>>>>>> --- a/drivers/scsi/scsi_lib.c
>>>>>>> +++ b/drivers/scsi/scsi_lib.c
>>>>>>> @@ -2638,6 +2638,40 @@ static int
>>>>>>> __scsi_internal_device_block_nowait(struct scsi_device
>>>>>>> *sdev)
>>>>>>>  	return 0;
>>>>>>>  }
>>>>>>>  
>>>>>>> +static DEFINE_SPINLOCK(sdev_queue_stop_lock);
>>>>>>> +
>>>>>>> +void scsi_start_queue(struct scsi_device *sdev)
>>>>>>> +{
>>>>>>> +	bool need_start;
>>>>>>> +	unsigned long flags;
>>>>>>> +
>>>>>>> +	spin_lock_irqsave(&sdev_queue_stop_lock, flags);
>>>>>>> +	need_start = sdev->queue_stopped;
>>>>>>> +	sdev->queue_stopped = 0;
>>>>>>> +	spin_unlock_irqrestore(&sdev_queue_stop_lock, flags);
>>>>>>> +
>>>>>>> +	if (need_start)
>>>>>>> +		blk_mq_unquiesce_queue(sdev->request_queue);
>>>>>>
>>>>>> Well, this is a classic atomic pattern:
>>>>>>
>>>>>> if (cmpxchg(&sdev->queue_stopped, 1, 0))
>>>>>> 	blk_mq_unquiesce_queue(sdev->request_queue);
>>>>>>
>>>>>> The reason to do it with atomics rather than spinlocks is
>>>>>>
>>>>>>    1. no need to disable interrupts: atomics are locked
>>>>>>    2. faster because a spinlock takes an exclusive line every
>>>>>> time but the
>>>>>>       read to check the value can be in shared mode in
>>>>>> cmpxchg
>>>>>>    3. it's just shorter and better code.
>>>>>>
>>>>>> The only minor downside is queue_stopped now needs to be a
>>>>>> u32.
>>>>>
>>>>> Are you fine with the change as-is, or do you want it redone? I
>>>>> can drop the SCSI parts and just queue up the dm fix.
>>>>> Personally I think it'd be better to get it fixed upfront.
>>>>
>>>> Well, given the path isn't hot, I don't really care.  However,
>>>> what I don't want is to have to continually bat back patches from
>>>> the make work code churners trying to update this code for being
>>>> the wrong pattern.  I think at the very least it needs a comment
>>>> saying why we chose a suboptimal pattern to try to forestall
>>>> this.
>>>
>>> Right, with a comment it's probably better. And as you said, since
>>> it's not a hot path, don't think we'd be revisiting it anyway.
>>>
>>> I'll amend the patch with a comment.
>>
>> I started adding the comment and took another look at this, and that
>> made me change my mind. We really should make this a cmpxcgh, it's
>> not even using a device lock here.
>>
>> I've dropped the two SCSI patches for now, Ming can you resend? If
>> James agrees, I really think queue_stopped should just have the type
>> changed and the patch redone with that using cmpxcgh().
> 
> Well, that's what I suggested originally, so I agree ... I don't think
> 31 more bytes is going to be a huge burden to scsi_device.
          ^^^^

Bits? :-)

-- 
Jens Axboe

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