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

From: Bob Liu
Date: Wed Oct 07 2015 - 06:35:00 EST



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@xxxxxxxxxx>
>> ---
>> 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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