Re: [PATCH] xen-blkback: defer freeing blkif to avoid blocking xenwatch

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 05/09/2014 07:34 PM, Valentin Priescu wrote:
> From: Valentin Priescu <priescuv@amazon.com>
>
> Currently xenwatch blocks in VBD disconnect, waiting for all pending I/O
> requests to finish. If the VBD is attached to a hot-swappable disk, then
> xenwatch can hang for a long period of time, stalling other watches.

Note that we have a GSoC student (copied here) who is going to be 
looking at xenwatch's single-threadness (?) over the summer.

-boris


>
>   INFO: task xenwatch:39 blocked for more than 120 seconds.
>   "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>   ffff880057f01bd0 0000000000000246 ffff880057f01ac0 ffffffff810b0782
>   ffff880057f01ad0 00000000000131c0 0000000000000004 ffff880057edb040
>   ffff8800344c6080 0000000000000000 ffff880058c00ba0 ffff880057edb040
>   Call Trace:
>   [<ffffffff810b0782>] ? irq_to_desc+0x12/0x20
>   [<ffffffff8128f761>] ? list_del+0x11/0x40
>   [<ffffffff8147a080>] ? wait_for_common+0x60/0x160
>   [<ffffffff8147bcef>] ? _raw_spin_lock_irqsave+0x2f/0x50
>   [<ffffffff8147bd49>] ? _raw_spin_unlock_irqrestore+0x19/0x20
>   [<ffffffff8147a26a>] schedule+0x3a/0x60
>   [<ffffffffa018fe6a>] xen_blkif_disconnect+0x8a/0x100 [xen_blkback]
>   [<ffffffff81079f70>] ? wake_up_bit+0x40/0x40
>   [<ffffffffa018ffce>] xen_blkbk_remove+0xae/0x1e0 [xen_blkback]
>   [<ffffffff8130b254>] xenbus_dev_remove+0x44/0x90
>   [<ffffffff81345cb7>] __device_release_driver+0x77/0xd0
>   [<ffffffff81346488>] device_release_driver+0x28/0x40
>   [<ffffffff813456e8>] bus_remove_device+0x78/0xe0
>   [<ffffffff81342c9f>] device_del+0x12f/0x1a0
>   [<ffffffff81342d2d>] device_unregister+0x1d/0x60
>   [<ffffffffa0190826>] frontend_changed+0xa6/0x4d0 [xen_blkback]
>   [<ffffffffa019c252>] ? frontend_changed+0x192/0x650 [xen_netback]
>   [<ffffffff8130ae50>] ? cmp_dev+0x60/0x60
>   [<ffffffff81344fe4>] ? bus_for_each_dev+0x94/0xa0
>   [<ffffffff8130b06e>] xenbus_otherend_changed+0xbe/0x120
>   [<ffffffff8130b4cb>] frontend_changed+0xb/0x10
>   [<ffffffff81309c82>] xenwatch_thread+0xf2/0x130
>   [<ffffffff81079f70>] ? wake_up_bit+0x40/0x40
>   [<ffffffff81309b90>] ? xenbus_directory+0x80/0x80
>   [<ffffffff810799d6>] kthread+0x96/0xa0
>   [<ffffffff81485934>] kernel_thread_helper+0x4/0x10
>   [<ffffffff814839f3>] ? int_ret_from_sys_call+0x7/0x1b
>   [<ffffffff8147c17c>] ? retint_restore_args+0x5/0x6
>   [<ffffffff81485930>] ? gs_change+0x13/0x13
>
> With this patch, when there is still pending I/O, the actual disconnect
> is done by the last reference holder (last pending I/O request). In this
> case, xenwatch doesn't block indefinitely.
>
> Signed-off-by: Valentin Priescu <priescuv@amazon.com>
> Reviewed-by: Steven Kady <stevkady@amazon.com>
> Reviewed-by: Steven Noonan <snoonan@amazon.com>
> Cc: Matt Wilson <msw@amazon.com>
> ---
>   drivers/block/xen-blkback/common.h |  4 +--
>   drivers/block/xen-blkback/xenbus.c | 60 ++++++++++++++++++++++++++------------
>   2 files changed, 43 insertions(+), 21 deletions(-)
>
> diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
> index be05277..f65b807 100644
> --- a/drivers/block/xen-blkback/common.h
> +++ b/drivers/block/xen-blkback/common.h
> @@ -314,7 +314,7 @@ struct xen_blkif {
>   	unsigned long long			st_rd_sect;
>   	unsigned long long			st_wr_sect;
>   
> -	wait_queue_head_t	waiting_to_free;
> +	struct work_struct	free_work;
>   	/* Thread shutdown wait queue. */
>   	wait_queue_head_t	shutdown_wq;
>   };
> @@ -361,7 +361,7 @@ struct pending_req {
>   #define xen_blkif_put(_b)				\
>   	do {						\
>   		if (atomic_dec_and_test(&(_b)->refcnt))	\
> -			wake_up(&(_b)->waiting_to_free);\
> +			schedule_work(&(_b)->free_work);\
>   	} while (0)
>   
>   struct phys_req {
> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
> index 9a547e6..7b6124e 100644
> --- a/drivers/block/xen-blkback/xenbus.c
> +++ b/drivers/block/xen-blkback/xenbus.c
> @@ -35,12 +35,26 @@ static void connect(struct backend_info *);
>   static int connect_ring(struct backend_info *);
>   static void backend_changed(struct xenbus_watch *, const char **,
>   			    unsigned int);
> +static void xen_blkif_free(struct xen_blkif *blkif);
> +static void xen_vbd_free(struct xen_vbd *vbd);
>   
>   struct xenbus_device *xen_blkbk_xenbus(struct backend_info *be)
>   {
>   	return be->dev;
>   }
>   
> +/*
> + * The last request could free the device from softirq context and
> + * xen_blkif_free() can sleep.
> + */
> +static void xen_blkif_deferred_free(struct work_struct *work)
> +{
> +	struct xen_blkif *blkif;
> +
> +	blkif = container_of(work, struct xen_blkif, free_work);
> +	xen_blkif_free(blkif);
> +}
> +
>   static int blkback_name(struct xen_blkif *blkif, char *buf)
>   {
>   	char *devpath, *devname;
> @@ -121,7 +135,6 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
>   	init_completion(&blkif->drain_complete);
>   	atomic_set(&blkif->drain, 0);
>   	blkif->st_print = jiffies;
> -	init_waitqueue_head(&blkif->waiting_to_free);
>   	blkif->persistent_gnts.rb_node = NULL;
>   	spin_lock_init(&blkif->free_pages_lock);
>   	INIT_LIST_HEAD(&blkif->free_pages);
> @@ -132,6 +145,7 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
>   	INIT_WORK(&blkif->persistent_purge_work, xen_blkbk_unmap_purged_grants);
>   
>   	INIT_LIST_HEAD(&blkif->pending_free);
> +	INIT_WORK(&blkif->free_work, xen_blkif_deferred_free);
>   
>   	for (i = 0; i < XEN_BLKIF_REQS; i++) {
>   		req = kzalloc(sizeof(*req), GFP_KERNEL);
> @@ -231,7 +245,7 @@ static int xen_blkif_map(struct xen_blkif *blkif, unsigned long shared_page,
>   	return 0;
>   }
>   
> -static void xen_blkif_disconnect(struct xen_blkif *blkif)
> +static int xen_blkif_disconnect(struct xen_blkif *blkif)
>   {
>   	if (blkif->xenblkd) {
>   		kthread_stop(blkif->xenblkd);
> @@ -239,19 +253,26 @@ static void xen_blkif_disconnect(struct xen_blkif *blkif)
>   		blkif->xenblkd = NULL;
>   	}
>   
> -	atomic_dec(&blkif->refcnt);
> -	wait_event(blkif->waiting_to_free, atomic_read(&blkif->refcnt) == 0);
> -	atomic_inc(&blkif->refcnt);
> +	/* The above kthread_stop() guarantees that at this point we
> +	 * don't have any discard_io or other_io requests. So, checking
> +	 * for inflight IO is enough.
> +	 */
> +	if (!atomic_read(&blkif->inflight)) {
> +		if (blkif->irq) {
> +			unbind_from_irqhandler(blkif->irq, blkif);
> +			blkif->irq = 0;
> +		}
>   
> -	if (blkif->irq) {
> -		unbind_from_irqhandler(blkif->irq, blkif);
> -		blkif->irq = 0;
> -	}
> +		if (blkif->blk_rings.common.sring) {
> +			xenbus_unmap_ring_vfree(blkif->be->dev,
> +				blkif->blk_ring);
> +			blkif->blk_rings.common.sring = NULL;
> +		}
>   
> -	if (blkif->blk_rings.common.sring) {
> -		xenbus_unmap_ring_vfree(blkif->be->dev, blkif->blk_ring);
> -		blkif->blk_rings.common.sring = NULL;
> +		return 0;
>   	}
> +
> +	return -EBUSY;
>   }
>   
>   static void xen_blkif_free(struct xen_blkif *blkif)
> @@ -259,8 +280,8 @@ static void xen_blkif_free(struct xen_blkif *blkif)
>   	struct pending_req *req, *n;
>   	int i = 0, j;
>   
> -	if (!atomic_dec_and_test(&blkif->refcnt))
> -		BUG();
> +	xen_blkif_disconnect(blkif);
> +	xen_vbd_free(&blkif->vbd);
>   
>   	/* Remove all persistent grants and the cache of ballooned pages. */
>   	xen_blkbk_free_caches(blkif);
> @@ -449,16 +470,15 @@ static int xen_blkbk_remove(struct xenbus_device *dev)
>   		be->backend_watch.node = NULL;
>   	}
>   
> +	dev_set_drvdata(&dev->dev, NULL);
> +
>   	if (be->blkif) {
>   		xen_blkif_disconnect(be->blkif);
> -		xen_vbd_free(&be->blkif->vbd);
> -		xen_blkif_free(be->blkif);
> -		be->blkif = NULL;
> +		xen_blkif_put(be->blkif);
>   	}
>   
>   	kfree(be->mode);
>   	kfree(be);
> -	dev_set_drvdata(&dev->dev, NULL);
>   	return 0;
>   }
>   
> @@ -700,7 +720,9 @@ static void frontend_changed(struct xenbus_device *dev,
>   		 * Enforce precondition before potential leak point.
>   		 * xen_blkif_disconnect() is idempotent.
>   		 */
> -		xen_blkif_disconnect(be->blkif);
> +		err = xen_blkif_disconnect(be->blkif);
> +		if (err)
> +			break;
>   
>   		err = connect_ring(be);
>   		if (err)