Re: [PATCH 1/1] [virt] virtio-blk: Use ida to allocate disk index

[Michael S. Tsirkin]

On Thu, Jun 09, 2011 at 06:41:56AM -0400, Mark Wu wrote:
> On 06/09/2011 05:14 AM, Tejun Heo wrote:
> > Hello,
> > 
> > On Thu, Jun 09, 2011 at 08:51:05AM +0930, Rusty Russell wrote:
> >> On Wed, 08 Jun 2011 09:08:29 -0400, Mark Wu <dwu@xxxxxxxxxx> wrote:
> >>> Hi Rusty,
> >>> Yes, I can't figure out an instance of disk probing in parallel either, but as
> >>> per the following commit, I think we still need use lock for safety. What's your opinion?
> >>>
> >>> commit 4034cc68157bfa0b6622efe368488d3d3e20f4e6
> >>> Author: Tejun Heo <tj@xxxxxxxxxx>
> >>> Date:   Sat Feb 21 11:04:45 2009 +0900
> >>>
> >>>     [SCSI] sd: revive sd_index_lock
> >>>
> >>>     Commit f27bac2761cab5a2e212dea602d22457a9aa6943 which converted sd to
> >>>     use ida instead of idr incorrectly removed sd_index_lock around id
> >>>     allocation and free.  idr/ida do have internal locks but they protect
> >>>     their free object lists not the allocation itself.  The caller is
> >>>     responsible for that.  This missing synchronization led to the same id
> >>>     being assigned to multiple devices leading to oops.
> >>
> >> I'm confused.  Tejun, Greg, anyone can probes happen in parallel?
> >>
> >> If so, I'll have to review all my drivers.
> > 
> > Unless async is explicitly used, probe happens sequentially.  IOW, if
> > there's no async_schedule() call, things won't happen in parallel.
> > That said, I think it wouldn't be such a bad idea to protect ida with
> > spinlock regardless unless the probe code explicitly requires
> > serialization.
> > 
> > Thanks.
> > 
> Since virtio blk driver doesn't use async probe, it needn't use spinlock to protect ida.
> So remove the lock from patch.
> 
> >From fbb396df9dbf8023f1b268be01b43529a3993d57 Mon Sep 17 00:00:00 2001
> From: Mark Wu <dwu@xxxxxxxxxx>
> Date: Thu, 9 Jun 2011 06:34:07 -0400
> Subject: [PATCH 1/1] [virt] virtio-blk: Use ida to allocate disk index
> 
> Current index allocation in virtio-blk is based on a monotonically
> increasing variable "index". It could cause some confusion about disk
> name in the case of hot-plugging disks. And it's impossible to find the
> lowest available index by just maintaining a simple index. So it's
> changed to use ida to allocate index via referring to the index
> allocation in scsi disk.
> 
> Signed-off-by: Mark Wu <dwu@xxxxxxxxxx>

Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>

This got lost in the noise and missed 3.1 which is unfortunate.
How about we apply this as is and look at cleanups as a next step?

> ---
>  drivers/block/virtio_blk.c |   28 +++++++++++++++++++++++-----
>  1 files changed, 23 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 079c088..bf81ab6 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -8,10 +8,13 @@
>  #include <linux/scatterlist.h>
>  #include <linux/string_helpers.h>
>  #include <scsi/scsi_cmnd.h>
> +#include <linux/idr.h>
>  
>  #define PART_BITS 4
>  
> -static int major, index;
> +static int major;
> +static DEFINE_IDA(vd_index_ida);
> +
>  struct workqueue_struct *virtblk_wq;
>  
>  struct virtio_blk
> @@ -23,6 +26,7 @@ struct virtio_blk
>  
>  	/* The disk structure for the kernel. */
>  	struct gendisk *disk;
> +	u32 index;
>  
>  	/* Request tracking. */
>  	struct list_head reqs;
> @@ -343,12 +347,23 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
>  	struct request_queue *q;
>  	int err;
>  	u64 cap;
> -	u32 v, blk_size, sg_elems, opt_io_size;
> +	u32 v, blk_size, sg_elems, opt_io_size, index;
>  	u16 min_io_size;
>  	u8 physical_block_exp, alignment_offset;
>  
> -	if (index_to_minor(index) >= 1 << MINORBITS)
> -		return -ENOSPC;
> +	do {
> +		if (!ida_pre_get(&vd_index_ida, GFP_KERNEL))
> +			return -ENOMEM;
> +		err = ida_get_new(&vd_index_ida, &index);
> +	} while (err == -EAGAIN);
> +
> +	if (err)
> +		return err;
> +
> +	if (index_to_minor(index) >= 1 << MINORBITS) {
> +		err =  -ENOSPC;
> +		goto out_free_index;
> +	}
>  
>  	/* We need to know how many segments before we allocate. */
>  	err = virtio_config_val(vdev, VIRTIO_BLK_F_SEG_MAX,
> @@ -421,7 +436,7 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
>  	vblk->disk->private_data = vblk;
>  	vblk->disk->fops = &virtblk_fops;
>  	vblk->disk->driverfs_dev = &vdev->dev;
> -	index++;
> +	vblk->index = index;
>  
>  	/* configure queue flush support */
>  	if (virtio_has_feature(vdev, VIRTIO_BLK_F_FLUSH))
> @@ -516,6 +531,8 @@ out_free_vq:
>  	vdev->config->del_vqs(vdev);
>  out_free_vblk:
>  	kfree(vblk);
> +out_free_index:
> +	ida_remove(&vd_index_ida, index);
>  out:
>  	return err;
>  }
> @@ -538,6 +555,7 @@ static void __devexit virtblk_remove(struct virtio_device *vdev)
>  	mempool_destroy(vblk->pool);
>  	vdev->config->del_vqs(vdev);
>  	kfree(vblk);
> +	ida_remove(&vd_index_ida, vblk->index);
>  }
>  
>  static const struct virtio_device_id id_table[] = {
> -- 
> 1.7.1
> 
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