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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH] xen-blkback: defer freeing blkif to avoid blocking xenwatch
On Sat, May 10, 2014 at 01:34:39AM +0200, Valentin Priescu wrote:
> From: Valentin Priescu <priescuv@xxxxxxxxxx>
>
> 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.
>
> 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@xxxxxxxxxx>
> Reviewed-by: Steven Kady <stevkady@xxxxxxxxxx>
> Reviewed-by: Steven Noonan <snoonan@xxxxxxxxxx>
> Cc: Matt Wilson <msw@xxxxxxxxxx>
Acked-by: Matt Wilson <msw@xxxxxxxxxx>
> ---
> 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)
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