linux-kernel - Re: [PATCH v4 04/10] xen/blkfront: split per device io

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Message-ID: <20151104015104.GA3882@x230.dumpdata.com>
Date:	Tue, 3 Nov 2015 20:51:05 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Bob Liu <bob.liu@...cle.com>
Cc:	xen-devel@...ts.xen.org, linux-kernel@...r.kernel.org,
	roger.pau@...rix.com, felipe.franciosi@...rix.com, axboe@...com,
	avanzini.arianna@...il.com, rafal.mielniczuk@...rix.com,
	jonathan.davies@...rix.com, david.vrabel@...rix.com
Subject: Re: [PATCH v4 04/10] xen/blkfront: split per device io_lock

On Wed, Nov 04, 2015 at 09:07:12AM +0800, Bob Liu wrote:
> 
> On 11/04/2015 04:09 AM, Konrad Rzeszutek Wilk wrote:
> > On Mon, Nov 02, 2015 at 12:21:40PM +0800, Bob Liu wrote:
> >> The per device io_lock became a coarser grained lock after multi-queues/rings
> >> was introduced, this patch introduced a fine-grained ring_lock for each ring.
> > 
> > s/was introduced/was introduced (see commit titled XYZ)/
> > 
> > s/introdued/introduces/
> >>
> >> The old io_lock was renamed to dev_lock and only protect the ->grants list
> > 
> > s/was/is/
> > s/protect/protects/
> > 
> >> which is shared by all rings.
> >>
> >> Signed-off-by: Bob Liu <bob.liu@...cle.com>
> >> ---
> >>  drivers/block/xen-blkfront.c | 57 ++++++++++++++++++++++++++------------------
> >>  1 file changed, 34 insertions(+), 23 deletions(-)
> >>
> >> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> >> index eab78e7..8cc5995 100644
> >> --- a/drivers/block/xen-blkfront.c
> >> +++ b/drivers/block/xen-blkfront.c
> >> @@ -121,6 +121,7 @@ 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;
> > 
> > Can you add a comment explaining the lock semantic? As in under what conditions
> > should it be taken? Like you have it below.
> > 
> >> +	spinlock_t ring_lock;
> >>  	unsigned int ring_ref[XENBUS_MAX_RING_PAGES];
> >>  	unsigned int evtchn, irq;
> >>  	struct work_struct work;
> >> @@ -138,7 +139,8 @@ struct blkfront_ring_info {
> >>   */
> >>  struct blkfront_info
> >>  {
> >> -	spinlock_t io_lock;
> >> +	/* Lock to proect info->grants list shared by multi rings */
> > 
> > s/proect/protect/
> > 
> > Missing full stop.
> > 
> >> +	spinlock_t dev_lock;
> > 
> > Shouldn't it be right next to what it is protecting?
> > 
> > That is right below (or above): 'struct list_head grants;'?
> > 
> >>  	struct mutex mutex;
> >>  	struct xenbus_device *xbdev;
> >>  	struct gendisk *gd;
> >> @@ -224,6 +226,7 @@ static int fill_grant_buffer(struct blkfront_ring_info *rinfo, int num)
> >>  	struct grant *gnt_list_entry, *n;
> >>  	int i = 0;
> >>  
> >> +	spin_lock_irq(&info->dev_lock);
> > 
> > Why there? Why not where you add it to the list?
> >>  	while(i < num) {
> >>  		gnt_list_entry = kzalloc(sizeof(struct grant), GFP_NOIO);
> >>  		if (!gnt_list_entry)
> >> @@ -242,6 +245,7 @@ static int fill_grant_buffer(struct blkfront_ring_info *rinfo, int num)
> >>  		list_add(&gnt_list_entry->node, &info->grants);
> > 
> > Right here that is?
> > 
> > You are holding the lock for the duration of 'kzalloc' and 'alloc_page'.
> > 
> > And more interestingly, GFP_NOIO translates to __GFP_WAIT which means
> > it can call 'schedule'. - And you have taken an spinlock. That should
> > have thrown lots of warnings?
> > 
> >>  		i++;
> >>  	}
> >> +	spin_unlock_irq(&info->dev_lock);
> >>  
> >>  	return 0;
> >>  
> >> @@ -254,6 +258,7 @@ out_of_memory:
> >>  		kfree(gnt_list_entry);
> >>  		i--;
> >>  	}
> >> +	spin_unlock_irq(&info->dev_lock);
> > 
> > Just do it around the 'list_del' operation. You are using an
> > 'safe'
> >>  	BUG_ON(i != 0);
> >>  	return -ENOMEM;
> >>  }
> >> @@ -265,6 +270,7 @@ static struct grant *get_grant(grant_ref_t *gref_head,
> >>  	struct grant *gnt_list_entry;
> >>  	unsigned long buffer_gfn;
> >>  
> >> +	spin_lock(&info->dev_lock);
> >>  	BUG_ON(list_empty(&info->grants));
> >>  	gnt_list_entry = list_first_entry(&info->grants, struct grant,
> >>  	                                  node);
> >> @@ -272,8 +278,10 @@ static struct grant *get_grant(grant_ref_t *gref_head,
> >>  
> >>  	if (gnt_list_entry->gref != GRANT_INVALID_REF) {
> >>  		info->persistent_gnts_c--;
> >> +		spin_unlock(&info->dev_lock);
> >>  		return gnt_list_entry;
> >>  	}
> >> +	spin_unlock(&info->dev_lock);
> > 
> > Just have one spin_unlock. Put it right before the 'if (gnt_list_entry->gref)..'.
> 
> That's used to protect info->persistent_gnts_c, will update all other place.

But you don't mention that in the description - that the lock is suppose
to also protect persistent_gnts_c. Please update that.

> 
> Thanks,
> -Bob
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