Re: [Xen-devel] [PATCH v4 04/10] xen/blkfront: split per device io_lock

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

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@xxxxxxxxxx>
> >> ---
> >>  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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