linux-kernel - Re: [PATCH v3 5/9] xen/blkfront: convert per device io_lock to per ring ring

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Date:	Wed, 07 Oct 2015 18:34:06 +0800
From:	Bob Liu <bob.liu@...cle.com>
To:	Roger Pau Monné <roger.pau@...rix.com>
CC:	xen-devel@...ts.xen.org, david.vrabel@...rix.com,
	linux-kernel@...r.kernel.org, konrad.wilk@...cle.com,
	felipe.franciosi@...rix.com, axboe@...com, hch@...radead.org,
	avanzini.arianna@...il.com, rafal.mielniczuk@...rix.com,
	boris.ostrovsky@...cle.com, jonathan.davies@...rix.com
Subject: Re: [PATCH v3 5/9] xen/blkfront: convert per device io_lock to per
 ring ring_lock


On 10/05/2015 10:13 PM, Roger Pau Monné wrote:
> El 05/09/15 a les 14.39, Bob Liu ha escrit:
>> The per device io_lock became a coarser grained lock after multi-queues/rings
>> was introduced, this patch converts it to a fine-grained per ring lock.
>>
>> NOTE: The per dev_info structure was no more protected by any lock.
> 
> I would rewrite this as:
> 
> Note that the per-device blkfront_dev_info structure is no longer
> protected by any lock.
> 

Will update.

>>
>> Signed-off-by: Bob Liu <bob.liu@...cle.com>
>> ---
>>  drivers/block/xen-blkfront.c |   44 +++++++++++++++++++-----------------------
>>  1 file changed, 20 insertions(+), 24 deletions(-)
>>
>> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
>> index bf45c99..1cae76b 100644
>> --- a/drivers/block/xen-blkfront.c
>> +++ b/drivers/block/xen-blkfront.c
>> @@ -123,6 +123,7 @@ MODULE_PARM_DESC(max_ring_page_order, "Maximum order of pages to be used for the
>>  struct blkfront_ring_info
>>  {
>>  	struct blkif_front_ring ring;
>> +	spinlock_t ring_lock;
>>  	unsigned int ring_ref[XENBUS_MAX_RING_PAGES];
>>  	unsigned int evtchn, irq;
>>  	struct work_struct work;
>> @@ -141,7 +142,6 @@ struct blkfront_ring_info
>>   * putting all kinds of interesting stuff here :-)
>>   */
>>  struct blkfront_dev_info {
>> -	spinlock_t io_lock;
>>  	struct mutex mutex;
>>  	struct xenbus_device *xbdev;
>>  	struct gendisk *gd;
>> @@ -637,29 +637,28 @@ static int blkif_queue_rq(struct blk_mq_hw_ctx *hctx,
>>  			   const struct blk_mq_queue_data *qd)
>>  {
>>  	struct blkfront_ring_info *rinfo = (struct blkfront_ring_info *)hctx->driver_data;
>> -	struct blkfront_dev_info *dinfo = rinfo->dinfo;
>>  
>>  	blk_mq_start_request(qd->rq);
>> -	spin_lock_irq(&dinfo->io_lock);
>> +	spin_lock_irq(&rinfo->ring_lock);
>>  	if (RING_FULL(&rinfo->ring))
>>  		goto out_busy;
>>  
>> -	if (blkif_request_flush_invalid(qd->rq, dinfo))
>> +	if (blkif_request_flush_invalid(qd->rq, rinfo->dinfo))
>>  		goto out_err;
>>  
>>  	if (blkif_queue_request(qd->rq, rinfo))
>>  		goto out_busy;
>>  
>>  	flush_requests(rinfo);
>> -	spin_unlock_irq(&dinfo->io_lock);
>> +	spin_unlock_irq(&rinfo->ring_lock);
>>  	return BLK_MQ_RQ_QUEUE_OK;
>>  
>>  out_err:
>> -	spin_unlock_irq(&dinfo->io_lock);
>> +	spin_unlock_irq(&rinfo->ring_lock);
>>  	return BLK_MQ_RQ_QUEUE_ERROR;
>>  
>>  out_busy:
>> -	spin_unlock_irq(&dinfo->io_lock);
>> +	spin_unlock_irq(&rinfo->ring_lock);
>>  	blk_mq_stop_hw_queue(hctx);
>>  	return BLK_MQ_RQ_QUEUE_BUSY;
>>  }
>> @@ -961,7 +960,7 @@ static void xlvbd_release_gendisk(struct blkfront_dev_info *dinfo)
>>  	dinfo->gd = NULL;
>>  }
>>  
>> -/* Must be called with io_lock holded */
>> +/* Must be called with ring_lock holded */
>                                     ^ held.
>>  static void kick_pending_request_queues(struct blkfront_ring_info *rinfo)
>>  {
>>  	if (!RING_FULL(&rinfo->ring))
>> @@ -972,10 +971,10 @@ static void blkif_restart_queue(struct work_struct *work)
>>  {
>>  	struct blkfront_ring_info *rinfo = container_of(work, struct blkfront_ring_info, work);
>>  
>> -	spin_lock_irq(&rinfo->dinfo->io_lock);
>> +	spin_lock_irq(&rinfo->ring_lock);
>>  	if (rinfo->dinfo->connected == BLKIF_STATE_CONNECTED)
>>  		kick_pending_request_queues(rinfo);
> 
> This seems wrong, why are you acquiring a per-ring lock in order to
> check a per-device field? IMHO, I think you need to introduce a
> per-device lock or drop the locking around this chunk if it's really not
> needed.
> 

The lock here is to protect kick_pending_request_queues() where we will check RING_FULL().
Will move this lock after the checking of per-device field.

Thanks,
-Bob

>> -	spin_unlock_irq(&rinfo->dinfo->io_lock);
>> +	spin_unlock_irq(&rinfo->ring_lock);
>>  }
>>  
--
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