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

[Valentin Priescu <vali.priescu@gmail.com>]

Yes, this patch applies to all kernels >= v3.14 (including the latest 
stable).

On 05/12/2014 08:42 AM, Vasiliy Tolstov wrote:
> As i understand this is needed for stable too?
>
> 2014-05-10 3:56 GMT+04:00 Matt Wilson <msw@linux.com>:
>> On Sat, May 10, 2014 at 01:34:39AM +0200, 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.
>>>
>>>   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>
>>
>> Acked-by: 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)
>>